<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2025 Workiva-->
<!--r:23dfce38-5000-4f6d-8aa5-7a8546c27f0c,g:3f5c2a85-2430-4f13-8f86-57062fb60287-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.lbfoster.com/role/CONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="simple" xlink:href="fstr-20241231.xsd#CONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/CONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_5c845478-d9c1-4d43-9f70-b68eeb22afff" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_StatementTable_5c845478-d9c1-4d43-9f70-b68eeb22afff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_6e82e0fc-ad5d-447e-8568-5f1578803dd2" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_5c845478-d9c1-4d43-9f70-b68eeb22afff" xlink:to="loc_srt_ProductOrServiceAxis_6e82e0fc-ad5d-447e-8568-5f1578803dd2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_6e82e0fc-ad5d-447e-8568-5f1578803dd2_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_6e82e0fc-ad5d-447e-8568-5f1578803dd2" xlink:to="loc_srt_ProductsAndServicesDomain_6e82e0fc-ad5d-447e-8568-5f1578803dd2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_b63b6b6a-6436-4ea7-ac3d-b2131a7bfd12" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_6e82e0fc-ad5d-447e-8568-5f1578803dd2" xlink:to="loc_srt_ProductsAndServicesDomain_b63b6b6a-6436-4ea7-ac3d-b2131a7bfd12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_9b9b3bac-d070-4170-84c4-a7ab397a0f59" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_b63b6b6a-6436-4ea7-ac3d-b2131a7bfd12" xlink:to="loc_us-gaap_ProductMember_9b9b3bac-d070-4170-84c4-a7ab397a0f59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_28cbae02-c93e-40ac-9eb1-9217769c91ef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_b63b6b6a-6436-4ea7-ac3d-b2131a7bfd12" xlink:to="loc_us-gaap_ServiceMember_28cbae02-c93e-40ac-9eb1-9217769c91ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_4f67cdfc-54ca-465b-a46f-1872e4a99339" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_4f67cdfc-54ca-465b-a46f-1872e4a99339" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_1ec500ca-3d66-41a5-a6b6-fa2a96d1ea84" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_CostOfRevenue_1ec500ca-3d66-41a5-a6b6-fa2a96d1ea84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_02786cbd-3679-479d-9753-15d21a3bd27c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GrossProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_GrossProfit_02786cbd-3679-479d-9753-15d21a3bd27c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_722438e8-5430-4c32-8ab5-b15fa3716629" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_722438e8-5430-4c32-8ab5-b15fa3716629" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_4dad2caa-7434-4a9e-95ac-871848b36ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_4dad2caa-7434-4a9e-95ac-871848b36ca6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_45485cbe-8035-44c4-9e16-65a45fa96465" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_45485cbe-8035-44c4-9e16-65a45fa96465" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_c6695a6b-1044-413c-804b-f7727d9f906c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_OperatingIncomeLoss_c6695a6b-1044-413c-804b-f7727d9f906c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_93d056d2-ba29-4c9e-9158-caab5bdc6449" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_93d056d2-ba29-4c9e-9158-caab5bdc6449" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_2452ab67-6c61-4e8a-aaff-48e0f38f77f5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_2452ab67-6c61-4e8a-aaff-48e0f38f77f5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_864f14f8-dee0-4fd3-aa87-774a58366b73" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_864f14f8-dee0-4fd3-aa87-774a58366b73" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_8248ade4-862d-40c9-bc92-0aa046208623" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_8248ade4-862d-40c9-bc92-0aa046208623" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_eaef7b87-c9a1-4fc8-b903-12d1a624c163" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_ProfitLoss_eaef7b87-c9a1-4fc8-b903-12d1a624c163" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_b6a61be9-cb3a-4e58-b1e9-d9eafdec446a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_b6a61be9-cb3a-4e58-b1e9-d9eafdec446a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_9508980f-3e9b-4574-99de-3a5f44b8fd0b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_NetIncomeLoss_9508980f-3e9b-4574-99de-3a5f44b8fd0b" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_36a10bf6-208d-42d9-a087-728b77f11ba8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_EarningsPerShareBasic_36a10bf6-208d-42d9-a087-728b77f11ba8" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_8e80c4db-3b87-43a3-8cfa-34cd7603417b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_EarningsPerShareDiluted_8e80c4db-3b87-43a3-8cfa-34cd7603417b" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_d8a26a54-e1ce-4b3f-9d6b-429b2315d2d3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_d8a26a54-e1ce-4b3f-9d6b-429b2315d2d3" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_48b99dd7-486e-4393-bac6-04fa86d59b13" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3a72d574-abec-463d-9153-6fb9699ded16" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_48b99dd7-486e-4393-bac6-04fa86d59b13" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="fstr-20241231.xsd#CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_28ba6e76-3944-4e29-ae66-472f5e4da105" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_9fdc2914-1cd0-46f8-bcd7-54eff7665b43" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_28ba6e76-3944-4e29-ae66-472f5e4da105" xlink:to="loc_us-gaap_StatementTable_9fdc2914-1cd0-46f8-bcd7-54eff7665b43" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_ca19b127-ed97-409a-8d67-a188e48f6e66" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_9fdc2914-1cd0-46f8-bcd7-54eff7665b43" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_ca19b127-ed97-409a-8d67-a188e48f6e66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ca19b127-ed97-409a-8d67-a188e48f6e66_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ca19b127-ed97-409a-8d67-a188e48f6e66" xlink:to="loc_us-gaap_EquityComponentDomain_ca19b127-ed97-409a-8d67-a188e48f6e66_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ca19b127-ed97-409a-8d67-a188e48f6e66" xlink:to="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_a4ddc438-1e40-4033-8a20-ed3a6476fb6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:to="loc_us-gaap_CommonStockMember_a4ddc438-1e40-4033-8a20-ed3a6476fb6c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_2d9a89e8-59d3-44e6-9726-78a1dbe6ce48" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_2d9a89e8-59d3-44e6-9726-78a1dbe6ce48" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_79acc134-d52f-4951-9420-ab80c83a83d3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:to="loc_us-gaap_RetainedEarningsMember_79acc134-d52f-4951-9420-ab80c83a83d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_c9034394-b17e-48cd-9b31-3db80cec0c7d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:to="loc_us-gaap_TreasuryStockCommonMember_c9034394-b17e-48cd-9b31-3db80cec0c7d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_ea1139da-31f0-4226-b832-8997b07a0f8c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_ea1139da-31f0-4226-b832-8997b07a0f8c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_9698ba40-6495-4b6f-86f5-ff2c70408f87" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_31312bd7-df0e-4c1d-ab54-ea82fb4a1f03" xlink:to="loc_us-gaap_NoncontrollingInterestMember_9698ba40-6495-4b6f-86f5-ff2c70408f87" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_28ba6e76-3944-4e29-ae66-472f5e4da105" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_d303967b-e9fb-4b0b-ace0-86c583f0f023" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_d303967b-e9fb-4b0b-ace0-86c583f0f023" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_00e90527-5b11-41a8-ab35-59334ffcd7a6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:to="loc_us-gaap_ProfitLoss_00e90527-5b11-41a8-ab35-59334ffcd7a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_03340838-0655-49e8-854e-f384324a455c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_03340838-0655-49e8-854e-f384324a455c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent_c28d1307-a616-4459-b0e7-b2935b7fca7e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_03340838-0655-49e8-854e-f384324a455c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent_c28d1307-a616-4459-b0e7-b2935b7fca7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_8084f161-2499-47d6-874c-fe115ac53de8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_03340838-0655-49e8-854e-f384324a455c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_8084f161-2499-47d6-874c-fe115ac53de8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_12c620cd-7970-40df-9742-2917169c09b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_03340838-0655-49e8-854e-f384324a455c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_12c620cd-7970-40df-9742-2917169c09b7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_5118bb11-1b4e-49e1-b870-4ceedb627abe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_5118bb11-1b4e-49e1-b870-4ceedb627abe" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_37bec907-b596-44c1-95d7-01a0d7027f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_37bec907-b596-44c1-95d7-01a0d7027f0d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_fd5a634e-3329-4abc-af83-17e2c9358c6f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_fd5a634e-3329-4abc-af83-17e2c9358c6f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance_5a3ea69b-5907-40d9-9780-ce8a3350d4d9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_8f1e3178-569a-4ceb-8095-530abb8d241b" xlink:to="loc_us-gaap_NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance_5a3ea69b-5907-40d9-9780-ce8a3350d4d9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_ad5cce9f-935d-4f6d-9d02-ddb91ce92bb6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#SummaryofSignificantAccountingPoliciesDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_1b380aa4-7638-41b8-9f94-bcc968753a97" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_1b380aa4-7638-41b8-9f94-bcc968753a97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_1b380aa4-7638-41b8-9f94-bcc968753a97_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_1b380aa4-7638-41b8-9f94-bcc968753a97" xlink:to="loc_us-gaap_SegmentDomain_1b380aa4-7638-41b8-9f94-bcc968753a97_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_722d7d87-d4af-427b-9c46-55d95f9888f1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_1b380aa4-7638-41b8-9f94-bcc968753a97" xlink:to="loc_us-gaap_SegmentDomain_722d7d87-d4af-427b-9c46-55d95f9888f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_02ac1d5d-104c-429f-bb25-569a7b128e8b" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_722d7d87-d4af-427b-9c46-55d95f9888f1" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_02ac1d5d-104c-429f-bb25-569a7b128e8b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitAxis_b6efb7a7-2b82-45ec-9961-741c5c3e57e7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReportingUnitAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:to="loc_us-gaap_ReportingUnitAxis_b6efb7a7-2b82-45ec-9961-741c5c3e57e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_b6efb7a7-2b82-45ec-9961-741c5c3e57e7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReportingUnitAxis_b6efb7a7-2b82-45ec-9961-741c5c3e57e7" xlink:to="loc_us-gaap_ReportingUnitDomain_b6efb7a7-2b82-45ec-9961-741c5c3e57e7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_5b5f59e2-80c9-44ec-b2d1-c1151a5c4ba6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReportingUnitAxis_b6efb7a7-2b82-45ec-9961-741c5c3e57e7" xlink:to="loc_us-gaap_ReportingUnitDomain_5b5f59e2-80c9-44ec-b2d1-c1151a5c4ba6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailProductsBusinessUnitMember_88333271-084d-4f69-992f-0c523171bfa9" xlink:href="fstr-20241231.xsd#fstr_RailProductsBusinessUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_5b5f59e2-80c9-44ec-b2d1-c1151a5c4ba6" xlink:to="loc_fstr_RailProductsBusinessUnitMember_88333271-084d-4f69-992f-0c523171bfa9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_CoatingsAndMeasurementMember_004ce3bf-7329-4c60-b6f4-05963266fbd0" xlink:href="fstr-20241231.xsd#fstr_CoatingsAndMeasurementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_5b5f59e2-80c9-44ec-b2d1-c1151a5c4ba6" xlink:to="loc_fstr_CoatingsAndMeasurementMember_004ce3bf-7329-4c60-b6f4-05963266fbd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_999e0b9a-e660-4c29-9956-1735f5aa03e4" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:to="loc_srt_StatementGeographicalAxis_999e0b9a-e660-4c29-9956-1735f5aa03e4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_999e0b9a-e660-4c29-9956-1735f5aa03e4_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_999e0b9a-e660-4c29-9956-1735f5aa03e4" xlink:to="loc_srt_SegmentGeographicalDomain_999e0b9a-e660-4c29-9956-1735f5aa03e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_efe809d8-f347-4cf8-a3a3-69952affae12" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_999e0b9a-e660-4c29-9956-1735f5aa03e4" xlink:to="loc_srt_SegmentGeographicalDomain_efe809d8-f347-4cf8-a3a3-69952affae12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_NonDomesticMember_e23e9393-6067-483f-b1fd-10ab99765832" xlink:href="fstr-20241231.xsd#fstr_NonDomesticMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_efe809d8-f347-4cf8-a3a3-69952affae12" xlink:to="loc_fstr_NonDomesticMember_e23e9393-6067-483f-b1fd-10ab99765832" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_57819bc7-80ad-4df5-aad0-6a8fb802eac0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_57819bc7-80ad-4df5-aad0-6a8fb802eac0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_57819bc7-80ad-4df5-aad0-6a8fb802eac0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_57819bc7-80ad-4df5-aad0-6a8fb802eac0" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_57819bc7-80ad-4df5-aad0-6a8fb802eac0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_29cfbe5e-ad52-4756-8d50-6cb8e979f022" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_57819bc7-80ad-4df5-aad0-6a8fb802eac0" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_29cfbe5e-ad52-4756-8d50-6cb8e979f022" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingMember_9643628a-9d84-42af-851f-4453ff609dbb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_29cfbe5e-ad52-4756-8d50-6cb8e979f022" xlink:to="loc_us-gaap_BuildingMember_9643628a-9d84-42af-851f-4453ff609dbb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_aa449db9-5246-466c-a6a7-d1707db5026f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_29cfbe5e-ad52-4756-8d50-6cb8e979f022" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_aa449db9-5246-466c-a6a7-d1707db5026f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_68003752-ace3-44da-a4d0-3e98e769f2dd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_29cfbe5e-ad52-4756-8d50-6cb8e979f022" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_68003752-ace3-44da-a4d0-3e98e769f2dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_33812d5e-8504-4dfb-a96a-4bbb511ea4f0" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:to="loc_srt_RangeAxis_33812d5e-8504-4dfb-a96a-4bbb511ea4f0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_33812d5e-8504-4dfb-a96a-4bbb511ea4f0_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_33812d5e-8504-4dfb-a96a-4bbb511ea4f0" xlink:to="loc_srt_RangeMember_33812d5e-8504-4dfb-a96a-4bbb511ea4f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_97dc884f-20e1-49a9-9371-195f65431a48" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_33812d5e-8504-4dfb-a96a-4bbb511ea4f0" xlink:to="loc_srt_RangeMember_97dc884f-20e1-49a9-9371-195f65431a48" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_bf46098d-d377-4c2d-97c0-3e3760232f29" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_97dc884f-20e1-49a9-9371-195f65431a48" xlink:to="loc_srt_MinimumMember_bf46098d-d377-4c2d-97c0-3e3760232f29" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_7d873f02-e8f2-4fea-866b-4059d1c0589d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_97dc884f-20e1-49a9-9371-195f65431a48" xlink:to="loc_srt_MaximumMember_7d873f02-e8f2-4fea-866b-4059d1c0589d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionAxis_2eb1cc6b-a56d-4dc5-b2ce-db873c9419e0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b01b6d39-b489-4412-9560-9f89b29533b1" xlink:to="loc_us-gaap_AssetAcquisitionAxis_2eb1cc6b-a56d-4dc5-b2ce-db873c9419e0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_2eb1cc6b-a56d-4dc5-b2ce-db873c9419e0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AssetAcquisitionAxis_2eb1cc6b-a56d-4dc5-b2ce-db873c9419e0" xlink:to="loc_us-gaap_AssetAcquisitionDomain_2eb1cc6b-a56d-4dc5-b2ce-db873c9419e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_e515f015-9228-4664-aa9d-02a95ce1334e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AssetAcquisitionAxis_2eb1cc6b-a56d-4dc5-b2ce-db873c9419e0" xlink:to="loc_us-gaap_AssetAcquisitionDomain_e515f015-9228-4664-aa9d-02a95ce1334e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_CougarMountainPrecastLLCMember_a61115e7-c56d-48a0-89a4-e77090ddeb8e" xlink:href="fstr-20241231.xsd#fstr_CougarMountainPrecastLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionDomain_e515f015-9228-4664-aa9d-02a95ce1334e" xlink:to="loc_fstr_CougarMountainPrecastLLCMember_a61115e7-c56d-48a0-89a4-e77090ddeb8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_6d920ebe-7c97-4ceb-8a79-a88c2373c874" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_NumberOfReportableSegments_6d920ebe-7c97-4ceb-8a79-a88c2373c874" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_228ca007-6210-404b-99b7-83ace6f00ee1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_NumberOfOperatingSegments_228ca007-6210-404b-99b7-83ace6f00ee1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_NumberOfManufacturingFacilities_1816ce8a-dd69-4dbf-a412-ea0581b15f0a" xlink:href="fstr-20241231.xsd#fstr_NumberOfManufacturingFacilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_fstr_NumberOfManufacturingFacilities_1816ce8a-dd69-4dbf-a412-ea0581b15f0a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionConsiderationTransferred_d840f0cb-8312-4eb3-9c45-7a6a53862cd2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionConsiderationTransferred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_AssetAcquisitionConsiderationTransferred_d840f0cb-8312-4eb3-9c45-7a6a53862cd2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfProductiveAssets_6111bc89-4ee1-4c14-9af9-df2e38c3cc77" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProceedsFromSaleOfProductiveAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_ProceedsFromSaleOfProductiveAssets_6111bc89-4ee1-4c14-9af9-df2e38c3cc77" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_3024dbcb-71a3-4215-afd4-f77c3d64e9f7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_3024dbcb-71a3-4215-afd4-f77c3d64e9f7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_b4d959ef-2f7b-4bc9-ba1d-3727f0d0c722" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_b4d959ef-2f7b-4bc9-ba1d-3727f0d0c722" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_1372d4bf-f4c7-4675-afa9-669fc1cc0aa7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_1372d4bf-f4c7-4675-afa9-669fc1cc0aa7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StandardProductWarrantyAccrual_2e5a328f-1869-45e2-a179-1670f4f7e001" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StandardProductWarrantyAccrual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_StandardProductWarrantyAccrual_2e5a328f-1869-45e2-a179-1670f4f7e001" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_5ad23c51-bfe0-4531-8d64-0324d1c6ae96" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_5ad23c51-bfe0-4531-8d64-0324d1c6ae96" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_9db40fdb-32db-4dcd-bc42-dd7b5a3674ec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_9db40fdb-32db-4dcd-bc42-dd7b5a3674ec" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_0f0d4fb1-0281-47cc-a61c-6cd131d00853" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7d16ede9-4623-477a-bcbb-61bcbf1c1c81" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_0f0d4fb1-0281-47cc-a61c-6cd131d00853" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/BusinessSegmentsNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#BusinessSegmentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/BusinessSegmentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitAxis_9acd9d46-ca3c-4017-9c9d-1532599563fd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReportingUnitAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_us-gaap_ReportingUnitAxis_9acd9d46-ca3c-4017-9c9d-1532599563fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_9acd9d46-ca3c-4017-9c9d-1532599563fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReportingUnitAxis_9acd9d46-ca3c-4017-9c9d-1532599563fd" xlink:to="loc_us-gaap_ReportingUnitDomain_9acd9d46-ca3c-4017-9c9d-1532599563fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_e3f13651-40df-4dfe-89c8-091204a8ece6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReportingUnitAxis_9acd9d46-ca3c-4017-9c9d-1532599563fd" xlink:to="loc_us-gaap_ReportingUnitDomain_e3f13651-40df-4dfe-89c8-091204a8ece6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_CoatingsAndMeasurementMember_b71331ff-a5dd-4b1c-a1cf-c3c385f87050" xlink:href="fstr-20241231.xsd#fstr_CoatingsAndMeasurementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_e3f13651-40df-4dfe-89c8-091204a8ece6" xlink:to="loc_fstr_CoatingsAndMeasurementMember_b71331ff-a5dd-4b1c-a1cf-c3c385f87050" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailProductsBusinessUnitMember_ff7543a2-fce7-4cbc-8dd1-44c03a2b7877" xlink:href="fstr-20241231.xsd#fstr_RailProductsBusinessUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_e3f13651-40df-4dfe-89c8-091204a8ece6" xlink:to="loc_fstr_RailProductsBusinessUnitMember_ff7543a2-fce7-4cbc-8dd1-44c03a2b7877" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionAxis_70f055f2-f408-45b8-aeb1-12268517fce2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_us-gaap_AssetAcquisitionAxis_70f055f2-f408-45b8-aeb1-12268517fce2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_70f055f2-f408-45b8-aeb1-12268517fce2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AssetAcquisitionAxis_70f055f2-f408-45b8-aeb1-12268517fce2" xlink:to="loc_us-gaap_AssetAcquisitionDomain_70f055f2-f408-45b8-aeb1-12268517fce2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_04e2586d-5e64-4ed8-811e-5a474d0cb5cf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AssetAcquisitionAxis_70f055f2-f408-45b8-aeb1-12268517fce2" xlink:to="loc_us-gaap_AssetAcquisitionDomain_04e2586d-5e64-4ed8-811e-5a474d0cb5cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_CougarMountainPrecastLLCMember_068c68fc-20a4-4174-a0d4-d0def79cf67c" xlink:href="fstr-20241231.xsd#fstr_CougarMountainPrecastLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionDomain_04e2586d-5e64-4ed8-811e-5a474d0cb5cf" xlink:to="loc_fstr_CougarMountainPrecastLLCMember_068c68fc-20a4-4174-a0d4-d0def79cf67c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_f7e7315d-72a6-464e-952c-349a0449c91c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_f7e7315d-72a6-464e-952c-349a0449c91c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_f7e7315d-72a6-464e-952c-349a0449c91c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_f7e7315d-72a6-464e-952c-349a0449c91c" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_f7e7315d-72a6-464e-952c-349a0449c91c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_a25fb426-4961-458e-93f2-0f75fe896c91" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_f7e7315d-72a6-464e-952c-349a0449c91c" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_a25fb426-4961-458e-93f2-0f75fe896c91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupNotDiscontinuedOperationsMember_2dc4ada0-7301-4c86-bd3a-a424332bfb3d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisposalGroupNotDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_a25fb426-4961-458e-93f2-0f75fe896c91" xlink:to="loc_us-gaap_DisposalGroupNotDiscontinuedOperationsMember_2dc4ada0-7301-4c86-bd3a-a424332bfb3d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1a33f9f1-c994-4ba3-82f4-940d73d51e31" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1a33f9f1-c994-4ba3-82f4-940d73d51e31" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_1a33f9f1-c994-4ba3-82f4-940d73d51e31_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1a33f9f1-c994-4ba3-82f4-940d73d51e31" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_1a33f9f1-c994-4ba3-82f4-940d73d51e31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_74619197-1186-4872-abae-042432975543" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1a33f9f1-c994-4ba3-82f4-940d73d51e31" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_74619197-1186-4872-abae-042432975543" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_BridgeGridDeckProductMember_7cbeca14-3d94-46e0-a4c9-504674f344c4" xlink:href="fstr-20241231.xsd#fstr_BridgeGridDeckProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_74619197-1186-4872-abae-042432975543" xlink:to="loc_fstr_BridgeGridDeckProductMember_7cbeca14-3d94-46e0-a4c9-504674f344c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_9affbcbb-11f3-4672-a42e-4096b0288775" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_srt_ConsolidationItemsAxis_9affbcbb-11f3-4672-a42e-4096b0288775" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_9affbcbb-11f3-4672-a42e-4096b0288775_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_9affbcbb-11f3-4672-a42e-4096b0288775" xlink:to="loc_srt_ConsolidationItemsDomain_9affbcbb-11f3-4672-a42e-4096b0288775_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_c2dacd49-1b39-4706-9f3f-6ff90b250b38" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_9affbcbb-11f3-4672-a42e-4096b0288775" xlink:to="loc_srt_ConsolidationItemsDomain_c2dacd49-1b39-4706-9f3f-6ff90b250b38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_86f53262-3d6d-45bb-b72d-a9a240d4a2e4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_c2dacd49-1b39-4706-9f3f-6ff90b250b38" xlink:to="loc_us-gaap_OperatingSegmentsMember_86f53262-3d6d-45bb-b72d-a9a240d4a2e4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_d1e4d4b7-a301-420a-891e-0c3b2a4c64ea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_d1e4d4b7-a301-420a-891e-0c3b2a4c64ea" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_d1e4d4b7-a301-420a-891e-0c3b2a4c64ea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_d1e4d4b7-a301-420a-891e-0c3b2a4c64ea" xlink:to="loc_us-gaap_SegmentDomain_d1e4d4b7-a301-420a-891e-0c3b2a4c64ea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_dba1662a-bf7d-403f-be49-2a73cda675d8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_d1e4d4b7-a301-420a-891e-0c3b2a4c64ea" xlink:to="loc_us-gaap_SegmentDomain_dba1662a-bf7d-403f-be49-2a73cda675d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_BridgeGridDeckProductMember_81a75d6e-5ea6-45de-805a-086a5d2f384f" xlink:href="fstr-20241231.xsd#fstr_BridgeGridDeckProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_dba1662a-bf7d-403f-be49-2a73cda675d8" xlink:to="loc_fstr_BridgeGridDeckProductMember_81a75d6e-5ea6-45de-805a-086a5d2f384f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_23266b0d-3e72-4f81-8bab-aaa4277cecbe" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_dba1662a-bf7d-403f-be49-2a73cda675d8" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_23266b0d-3e72-4f81-8bab-aaa4277cecbe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesAndServicesSegmentMember_a521ba5a-9911-4752-9e78-412f863de501" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesAndServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_dba1662a-bf7d-403f-be49-2a73cda675d8" xlink:to="loc_fstr_RailTechnologiesAndServicesSegmentMember_a521ba5a-9911-4752-9e78-412f863de501" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_117ecae1-b9e0-4e92-b7a3-1b559edd460b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_117ecae1-b9e0-4e92-b7a3-1b559edd460b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_117ecae1-b9e0-4e92-b7a3-1b559edd460b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_117ecae1-b9e0-4e92-b7a3-1b559edd460b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_117ecae1-b9e0-4e92-b7a3-1b559edd460b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_cb7d34d8-cd04-4676-bc49-3bec3bf3d5cf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_117ecae1-b9e0-4e92-b7a3-1b559edd460b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_cb7d34d8-cd04-4676-bc49-3bec3bf3d5cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_706ba29d-26b9-4924-bb9f-a01089e91ded" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_cb7d34d8-cd04-4676-bc49-3bec3bf3d5cf" xlink:to="loc_us-gaap_CostOfSalesMember_706ba29d-26b9-4924-bb9f-a01089e91ded" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_b38d3c42-f1b0-40bb-a2f4-53dcaa489bd9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_cb7d34d8-cd04-4676-bc49-3bec3bf3d5cf" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_b38d3c42-f1b0-40bb-a2f4-53dcaa489bd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesMember_9d827fa3-e049-43cb-b5b9-614d9bb3b370" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_cb7d34d8-cd04-4676-bc49-3bec3bf3d5cf" xlink:to="loc_us-gaap_SalesMember_9d827fa3-e049-43cb-b5b9-614d9bb3b370" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_GrossProfitMember_d8c7608e-af34-4322-9a8a-7919dc01e9eb" xlink:href="fstr-20241231.xsd#fstr_GrossProfitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_cb7d34d8-cd04-4676-bc49-3bec3bf3d5cf" xlink:to="loc_fstr_GrossProfitMember_d8c7608e-af34-4322-9a8a-7919dc01e9eb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_89449dbd-8bfd-41b4-b298-a2869e71a322" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_srt_MajorCustomersAxis_89449dbd-8bfd-41b4-b298-a2869e71a322" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_89449dbd-8bfd-41b4-b298-a2869e71a322_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_89449dbd-8bfd-41b4-b298-a2869e71a322" xlink:to="loc_srt_NameOfMajorCustomerDomain_89449dbd-8bfd-41b4-b298-a2869e71a322_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_0df5a1f6-0ed0-4f59-b9a8-a8fcb87e7433" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_89449dbd-8bfd-41b4-b298-a2869e71a322" xlink:to="loc_srt_NameOfMajorCustomerDomain_0df5a1f6-0ed0-4f59-b9a8-a8fcb87e7433" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OneCustomerMember_3a00c943-e6f9-489b-8c08-6a48e9ff084a" xlink:href="fstr-20241231.xsd#fstr_OneCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_0df5a1f6-0ed0-4f59-b9a8-a8fcb87e7433" xlink:to="loc_fstr_OneCustomerMember_3a00c943-e6f9-489b-8c08-6a48e9ff084a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_54855053-42c1-4c76-ab97-f2455f9734b3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_3433e3b5-9e41-42da-b20e-1eefe580feeb" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_54855053-42c1-4c76-ab97-f2455f9734b3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_54855053-42c1-4c76-ab97-f2455f9734b3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_54855053-42c1-4c76-ab97-f2455f9734b3" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_54855053-42c1-4c76-ab97-f2455f9734b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_70bea448-3a99-4584-8c33-cd697036bbc8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_54855053-42c1-4c76-ab97-f2455f9734b3" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_70bea448-3a99-4584-8c33-cd697036bbc8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_11049d8f-d4ea-458e-8648-07250c640cdf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_70bea448-3a99-4584-8c33-cd697036bbc8" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_11049d8f-d4ea-458e-8648-07250c640cdf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_eada647b-a527-4aff-85e2-1b1d9d96ea2f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_NumberOfOperatingSegments_eada647b-a527-4aff-85e2-1b1d9d96ea2f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_5ae42c3a-8d0d-47aa-bef1-4b13ac248fac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_NumberOfReportableSegments_5ae42c3a-8d0d-47aa-bef1-4b13ac248fac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_7221fa47-c5fc-4a13-8355-204baafb3550" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProceedsFromDivestitureOfBusinesses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_7221fa47-c5fc-4a13-8355-204baafb3550" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_258312cf-1c7b-4150-bce2-fb9b4a8b1e3e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_258312cf-1c7b-4150-bce2-fb9b4a8b1e3e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionConsiderationTransferred_3db55f04-b9e6-4e18-8a9f-09d6559b4a12" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionConsiderationTransferred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_AssetAcquisitionConsiderationTransferred_3db55f04-b9e6-4e18-8a9f-09d6559b4a12" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_fe5142d6-817b-4003-bf32-6e01f880d515" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_fe5142d6-817b-4003-bf32-6e01f880d515" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnDispositionOfAssets1_6f3b0747-e4d4-4f47-88ae-e790b2deefee" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnDispositionOfAssets1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_GainLossOnDispositionOfAssets1_6f3b0747-e4d4-4f47-88ae-e790b2deefee" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryWriteDown_70806745-13cf-4f77-9aef-cb7a6b9720e7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InventoryWriteDown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_InventoryWriteDown_70806745-13cf-4f77-9aef-cb7a6b9720e7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_PersonnelExpenses_9a54addc-f178-489b-bc04-61070748d9d7" xlink:href="fstr-20241231.xsd#fstr_PersonnelExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_fstr_PersonnelExpenses_9a54addc-f178-489b-bc04-61070748d9d7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OtherExitCosts_39dc547e-8d05-4268-8ed1-1a49dd0521e8" xlink:href="fstr-20241231.xsd#fstr_OtherExitCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_fstr_OtherExitCosts_39dc547e-8d05-4268-8ed1-1a49dd0521e8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice_01c37347-2491-4ca6-ae2d-ce5e2bd063f8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice_01c37347-2491-4ca6-ae2d-ce5e2bd063f8" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_8c68d0d1-5e43-4b5a-975b-7a5e99dbbf1b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_RestructuringCharges_8c68d0d1-5e43-4b5a-975b-7a5e99dbbf1b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_97172ee7-8c44-49c5-b89f-d3c604d0e029" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_97172ee7-8c44-49c5-b89f-d3c604d0e029" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_8b64a94c-e2f1-4a76-b604-8b9ac48abc19" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProvisionForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_8b64a94c-e2f1-4a76-b604-8b9ac48abc19" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCosts_e0f4e44b-28be-4887-b190-7466840aef76" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_ce5953e5-d201-41f4-826d-ebe641c5bbb0" xlink:to="loc_us-gaap_RestructuringCosts_e0f4e44b-28be-4887-b190-7466840aef76" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/BusinessSegmentsScheduleofReconciliationofOperatingIncometoIncomeBeforeTaxesDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#BusinessSegmentsScheduleofReconciliationofOperatingIncometoIncomeBeforeTaxesDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/BusinessSegmentsScheduleofReconciliationofOperatingIncometoIncomeBeforeTaxesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4b9a7f38-3f8c-4939-a726-dca0660727e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4b9a7f38-3f8c-4939-a726-dca0660727e5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_c561069c-5e86-4e62-a94a-36eccfc2246a" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4b9a7f38-3f8c-4939-a726-dca0660727e5" xlink:to="loc_srt_ConsolidationItemsAxis_c561069c-5e86-4e62-a94a-36eccfc2246a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_c561069c-5e86-4e62-a94a-36eccfc2246a_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_c561069c-5e86-4e62-a94a-36eccfc2246a" xlink:to="loc_srt_ConsolidationItemsDomain_c561069c-5e86-4e62-a94a-36eccfc2246a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_3ce7095d-b1fb-4771-9902-c43d9e84a93d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_c561069c-5e86-4e62-a94a-36eccfc2246a" xlink:to="loc_srt_ConsolidationItemsDomain_3ce7095d-b1fb-4771-9902-c43d9e84a93d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_3304d59c-8306-465b-8b32-1c85a3e98d81" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_3ce7095d-b1fb-4771-9902-c43d9e84a93d" xlink:to="loc_us-gaap_OperatingSegmentsMember_3304d59c-8306-465b-8b32-1c85a3e98d81" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_CorporateAndReconcilingItemsMember_06754191-9368-4a14-bbbe-bbb60a797350" xlink:href="fstr-20241231.xsd#fstr_CorporateAndReconcilingItemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_3ce7095d-b1fb-4771-9902-c43d9e84a93d" xlink:to="loc_fstr_CorporateAndReconcilingItemsMember_06754191-9368-4a14-bbbe-bbb60a797350" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_67673560-d438-491f-be53-b5ee1902dadb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4b9a7f38-3f8c-4939-a726-dca0660727e5" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_67673560-d438-491f-be53-b5ee1902dadb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_67673560-d438-491f-be53-b5ee1902dadb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_67673560-d438-491f-be53-b5ee1902dadb" xlink:to="loc_us-gaap_SegmentDomain_67673560-d438-491f-be53-b5ee1902dadb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_8b1ca7d6-2d81-4ad5-bba8-763676a35c14" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_67673560-d438-491f-be53-b5ee1902dadb" xlink:to="loc_us-gaap_SegmentDomain_8b1ca7d6-2d81-4ad5-bba8-763676a35c14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesAndServicesSegmentMember_52c61f56-110f-4c95-a69c-a2759a6f9dc6" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesAndServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_8b1ca7d6-2d81-4ad5-bba8-763676a35c14" xlink:to="loc_fstr_RailTechnologiesAndServicesSegmentMember_52c61f56-110f-4c95-a69c-a2759a6f9dc6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_d407bfa9-4bf7-475e-8178-04ad61bf3eba" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_8b1ca7d6-2d81-4ad5-bba8-763676a35c14" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_d407bfa9-4bf7-475e-8178-04ad61bf3eba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_d8ffdf52-c53f-49fa-8352-e4559a1350ac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_d8ffdf52-c53f-49fa-8352-e4559a1350ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_28da8663-59db-4fc5-8418-0fe2ea056fde" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_OperatingExpensesAbstract_28da8663-59db-4fc5-8418-0fe2ea056fde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_08dff01b-9cbd-4c45-9901-67d83940345f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_28da8663-59db-4fc5-8418-0fe2ea056fde" xlink:to="loc_us-gaap_CostOfRevenue_08dff01b-9cbd-4c45-9901-67d83940345f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LaborAndRelatedExpense_5a4b79bd-7fdd-4742-a18f-4bb84d99fed6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LaborAndRelatedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_28da8663-59db-4fc5-8418-0fe2ea056fde" xlink:to="loc_us-gaap_LaborAndRelatedExpense_5a4b79bd-7fdd-4742-a18f-4bb84d99fed6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_PurchasedServicesExpense_24a3e1f4-a400-4dfa-9242-3e0ff8966f65" xlink:href="fstr-20241231.xsd#fstr_PurchasedServicesExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_28da8663-59db-4fc5-8418-0fe2ea056fde" xlink:to="loc_fstr_PurchasedServicesExpense_24a3e1f4-a400-4dfa-9242-3e0ff8966f65" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_877f4e2a-81f9-42d6-9404-16f33ec45a93" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_28da8663-59db-4fc5-8418-0fe2ea056fde" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_877f4e2a-81f9-42d6-9404-16f33ec45a93" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentForAmortization_20f971c5-ed12-444e-a131-fa2d7c8ba555" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentForAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_28da8663-59db-4fc5-8418-0fe2ea056fde" xlink:to="loc_us-gaap_AdjustmentForAmortization_20f971c5-ed12-444e-a131-fa2d7c8ba555" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_fc2b5901-cba9-437b-9913-51dc256c1227" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_OperatingIncomeLoss_fc2b5901-cba9-437b-9913-51dc256c1227" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_e44f64fc-3c2f-4a6e-9382-d29d4b7bb0ff" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_e44f64fc-3c2f-4a6e-9382-d29d4b7bb0ff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_7d2daf31-75d2-4988-9c58-da09decb7e2f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_7d2daf31-75d2-4988-9c58-da09decb7e2f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_1cdc5ad1-6c9d-4abc-bb9b-86bfaba2ece9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_1cdc5ad1-6c9d-4abc-bb9b-86bfaba2ece9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_PublicCompanyCosts_af4246d5-c269-4f0b-ae6e-0a4d5c3f1127" xlink:href="fstr-20241231.xsd#fstr_PublicCompanyCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_fstr_PublicCompanyCosts_af4246d5-c269-4f0b-ae6e-0a4d5c3f1127" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_CorporateExecutiveManagementCosts_574fce5b-d31b-4d69-a842-64462cbaa2c7" xlink:href="fstr-20241231.xsd#fstr_CorporateExecutiveManagementCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_fstr_CorporateExecutiveManagementCosts_574fce5b-d31b-4d69-a842-64462cbaa2c7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_CorporateManagementStockBasedCompensation_8c020dda-8211-4e76-830b-a9869b84efbc" xlink:href="fstr-20241231.xsd#fstr_CorporateManagementStockBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_fstr_CorporateManagementStockBasedCompensation_8c020dda-8211-4e76-830b-a9869b84efbc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OtherCorporateCharges_0728fcfe-b862-4b51-87c0-60a773c51d1c" xlink:href="fstr-20241231.xsd#fstr_OtherCorporateCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_fstr_OtherCorporateCharges_0728fcfe-b862-4b51-87c0-60a773c51d1c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_201d039c-1738-4339-9888-94bfcd071979" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_09de0fd6-d1ea-4a58-85d0-b8ccde6484d3" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_201d039c-1738-4339-9888-94bfcd071979" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/BusinessSegmentsScheduleofAssetReportableSegmentDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#BusinessSegmentsScheduleofAssetReportableSegmentDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/BusinessSegmentsScheduleofAssetReportableSegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_de5b7b39-8db9-491b-8d9d-0fc29f39286a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41506a2c-9840-4509-b58b-fc4d2c7d2f09" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_de5b7b39-8db9-491b-8d9d-0fc29f39286a" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41506a2c-9840-4509-b58b-fc4d2c7d2f09" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_c0b2244d-2224-4ac0-a38a-103e5d75ec4c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41506a2c-9840-4509-b58b-fc4d2c7d2f09" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_c0b2244d-2224-4ac0-a38a-103e5d75ec4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_c0b2244d-2224-4ac0-a38a-103e5d75ec4c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_c0b2244d-2224-4ac0-a38a-103e5d75ec4c" xlink:to="loc_us-gaap_SegmentDomain_c0b2244d-2224-4ac0-a38a-103e5d75ec4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_0fece3e2-320d-4bc5-b7f5-4ce86532a740" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_c0b2244d-2224-4ac0-a38a-103e5d75ec4c" xlink:to="loc_us-gaap_SegmentDomain_0fece3e2-320d-4bc5-b7f5-4ce86532a740" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesAndServicesSegmentMember_1113af8a-632e-43bf-b225-e393ea0f5267" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesAndServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_0fece3e2-320d-4bc5-b7f5-4ce86532a740" xlink:to="loc_fstr_RailTechnologiesAndServicesSegmentMember_1113af8a-632e-43bf-b225-e393ea0f5267" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_bc7b7b39-e5b1-41fa-941b-fe2d60af409b" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_0fece3e2-320d-4bc5-b7f5-4ce86532a740" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_bc7b7b39-e5b1-41fa-941b-fe2d60af409b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_b12d31c3-73a5-408d-946f-14c95d961f11" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41506a2c-9840-4509-b58b-fc4d2c7d2f09" xlink:to="loc_srt_ConsolidationItemsAxis_b12d31c3-73a5-408d-946f-14c95d961f11" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_b12d31c3-73a5-408d-946f-14c95d961f11_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_b12d31c3-73a5-408d-946f-14c95d961f11" xlink:to="loc_srt_ConsolidationItemsDomain_b12d31c3-73a5-408d-946f-14c95d961f11_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_b6fa7537-a338-4b24-9910-90b2ae39837c" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_b12d31c3-73a5-408d-946f-14c95d961f11" xlink:to="loc_srt_ConsolidationItemsDomain_b6fa7537-a338-4b24-9910-90b2ae39837c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_dfb4d50f-38ff-4525-91c1-f6cd9b810aa1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_b6fa7537-a338-4b24-9910-90b2ae39837c" xlink:to="loc_us-gaap_OperatingSegmentsMember_dfb4d50f-38ff-4525-91c1-f6cd9b810aa1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_614801ed-0437-47e8-a58f-40cbd7276c98" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_b6fa7537-a338-4b24-9910-90b2ae39837c" xlink:to="loc_us-gaap_CorporateNonSegmentMember_614801ed-0437-47e8-a58f-40cbd7276c98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_1774d1e0-4109-43cb-b0e1-0f3dd546147c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_de5b7b39-8db9-491b-8d9d-0fc29f39286a" xlink:to="loc_us-gaap_Assets_1774d1e0-4109-43cb-b0e1-0f3dd546147c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_57bc6f13-4831-4c3d-9868-91188e44b703" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_de5b7b39-8db9-491b-8d9d-0fc29f39286a" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_57bc6f13-4831-4c3d-9868-91188e44b703" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_42ef8925-4770-4fb9-87d4-a7c9bf0a1d7c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_de5b7b39-8db9-491b-8d9d-0fc29f39286a" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_42ef8925-4770-4fb9-87d4-a7c9bf0a1d7c" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/BusinessSegmentsScheduleofSalesbyGeographicalLocationDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#BusinessSegmentsScheduleofSalesbyGeographicalLocationDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/BusinessSegmentsScheduleofSalesbyGeographicalLocationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0b68fc4f-b95b-401f-9c04-9c86f377489c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_d67ab624-fcf9-446c-98c0-48c9095f02c0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0b68fc4f-b95b-401f-9c04-9c86f377489c" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_d67ab624-fcf9-446c-98c0-48c9095f02c0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_466f21e3-cf02-4764-a230-0a624f157939" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_d67ab624-fcf9-446c-98c0-48c9095f02c0" xlink:to="loc_srt_StatementGeographicalAxis_466f21e3-cf02-4764-a230-0a624f157939" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_466f21e3-cf02-4764-a230-0a624f157939_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_466f21e3-cf02-4764-a230-0a624f157939" xlink:to="loc_srt_SegmentGeographicalDomain_466f21e3-cf02-4764-a230-0a624f157939_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_1279d819-0e28-4457-b81d-894621d02774" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_466f21e3-cf02-4764-a230-0a624f157939" xlink:to="loc_srt_SegmentGeographicalDomain_1279d819-0e28-4457-b81d-894621d02774" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_72360d4a-dd47-4ac0-927d-921e9b9e1744" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_1279d819-0e28-4457-b81d-894621d02774" xlink:to="loc_country_US_72360d4a-dd47-4ac0-927d-921e9b9e1744" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_da22b059-fd1e-4cec-b4fd-3df7d14fd349" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_1279d819-0e28-4457-b81d-894621d02774" xlink:to="loc_country_CA_da22b059-fd1e-4cec-b4fd-3df7d14fd349" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_d1926b84-3b52-4186-9f74-a7e0a1f88e67" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_1279d819-0e28-4457-b81d-894621d02774" xlink:to="loc_country_GB_d1926b84-3b52-4186-9f74-a7e0a1f88e67" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OtherGeographicalLocationsMember_7cbd4d55-aab2-4b07-b5e5-2c98ad734050" xlink:href="fstr-20241231.xsd#fstr_OtherGeographicalLocationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_1279d819-0e28-4457-b81d-894621d02774" xlink:to="loc_fstr_OtherGeographicalLocationsMember_7cbd4d55-aab2-4b07-b5e5-2c98ad734050" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_3ee06bc3-7f4e-4b97-9e1b-4f2d86727a6e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0b68fc4f-b95b-401f-9c04-9c86f377489c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_3ee06bc3-7f4e-4b97-9e1b-4f2d86727a6e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/BusinessSegmentsScheduleofLongLivedAssetsByGeographicalLocationDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#BusinessSegmentsScheduleofLongLivedAssetsByGeographicalLocationDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/BusinessSegmentsScheduleofLongLivedAssetsByGeographicalLocationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0407ac59-cfef-406a-8c2a-89c9fa7fa9bd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_37b79928-0939-42a1-9d26-84a3558a9e60" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0407ac59-cfef-406a-8c2a-89c9fa7fa9bd" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_37b79928-0939-42a1-9d26-84a3558a9e60" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_378324ee-be3c-4203-8eaa-376c8bfaf87b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_37b79928-0939-42a1-9d26-84a3558a9e60" xlink:to="loc_srt_StatementGeographicalAxis_378324ee-be3c-4203-8eaa-376c8bfaf87b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_378324ee-be3c-4203-8eaa-376c8bfaf87b_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_378324ee-be3c-4203-8eaa-376c8bfaf87b" xlink:to="loc_srt_SegmentGeographicalDomain_378324ee-be3c-4203-8eaa-376c8bfaf87b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_d5bfde3e-306c-483b-8ad1-913be25af473" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_378324ee-be3c-4203-8eaa-376c8bfaf87b" xlink:to="loc_srt_SegmentGeographicalDomain_d5bfde3e-306c-483b-8ad1-913be25af473" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_a984c90e-7aef-4f6a-8083-36a418244b16" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5bfde3e-306c-483b-8ad1-913be25af473" xlink:to="loc_country_US_a984c90e-7aef-4f6a-8083-36a418244b16" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_a8054567-b5b3-4fe3-8812-4d71a787e215" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5bfde3e-306c-483b-8ad1-913be25af473" xlink:to="loc_country_CA_a8054567-b5b3-4fe3-8812-4d71a787e215" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_8afe48d9-2cc4-4aac-9caa-80065fc835ef" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5bfde3e-306c-483b-8ad1-913be25af473" xlink:to="loc_country_GB_8afe48d9-2cc4-4aac-9caa-80065fc835ef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OtherGeographicalLocationsMember_4da4d98c-6d4d-4e23-81af-12ff43b7e037" xlink:href="fstr-20241231.xsd#fstr_OtherGeographicalLocationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d5bfde3e-306c-483b-8ad1-913be25af473" xlink:to="loc_fstr_OtherGeographicalLocationsMember_4da4d98c-6d4d-4e23-81af-12ff43b7e037" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_d467f8db-87fd-4cd8-b304-8e13e56e3674" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_0407ac59-cfef-406a-8c2a-89c9fa7fa9bd" xlink:to="loc_us-gaap_NoncurrentAssets_d467f8db-87fd-4cd8-b304-8e13e56e3674" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/BusinessSegmentsScheduleofSalesbyMajorProductLineDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#BusinessSegmentsScheduleofSalesbyMajorProductLineDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/BusinessSegmentsScheduleofSalesbyMajorProductLineDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_f7c07cb8-91b8-46fe-9e6e-386504886d1e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_1edc6bc0-f121-4633-a474-1397ccbfa91d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_f7c07cb8-91b8-46fe-9e6e-386504886d1e" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_1edc6bc0-f121-4633-a474-1397ccbfa91d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_317b27ac-536f-4f11-8fe9-45a9301a9f39" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_1edc6bc0-f121-4633-a474-1397ccbfa91d" xlink:to="loc_srt_ProductOrServiceAxis_317b27ac-536f-4f11-8fe9-45a9301a9f39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_317b27ac-536f-4f11-8fe9-45a9301a9f39_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_317b27ac-536f-4f11-8fe9-45a9301a9f39" xlink:to="loc_srt_ProductsAndServicesDomain_317b27ac-536f-4f11-8fe9-45a9301a9f39_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_579d3a40-7ec3-4ae7-ba42-1fee48abcbaf" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_317b27ac-536f-4f11-8fe9-45a9301a9f39" xlink:to="loc_srt_ProductsAndServicesDomain_579d3a40-7ec3-4ae7-ba42-1fee48abcbaf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailProductsMember_204e0f4e-d094-42f0-bf95-81c093640cd9" xlink:href="fstr-20241231.xsd#fstr_RailProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_579d3a40-7ec3-4ae7-ba42-1fee48abcbaf" xlink:to="loc_fstr_RailProductsMember_204e0f4e-d094-42f0-bf95-81c093640cd9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailGlobalFrictionManagementMember_bf0c71d4-b9d2-43e8-80cb-b86bf7df662e" xlink:href="fstr-20241231.xsd#fstr_RailGlobalFrictionManagementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_579d3a40-7ec3-4ae7-ba42-1fee48abcbaf" xlink:to="loc_fstr_RailGlobalFrictionManagementMember_bf0c71d4-b9d2-43e8-80cb-b86bf7df662e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesProductsMember_b97cafd7-c80a-4ca3-99cc-f9579679a65f" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_579d3a40-7ec3-4ae7-ba42-1fee48abcbaf" xlink:to="loc_fstr_RailTechnologiesProductsMember_b97cafd7-c80a-4ca3-99cc-f9579679a65f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_PrecastConcreteBuildingsMember_9be1f368-1529-401d-bfd9-edb0c4b2fd01" xlink:href="fstr-20241231.xsd#fstr_PrecastConcreteBuildingsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_579d3a40-7ec3-4ae7-ba42-1fee48abcbaf" xlink:to="loc_fstr_PrecastConcreteBuildingsMember_9be1f368-1529-401d-bfd9-edb0c4b2fd01" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_SteelProductsMember_7e3117e5-5667-4678-bf73-269e6cf447c1" xlink:href="fstr-20241231.xsd#fstr_SteelProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_579d3a40-7ec3-4ae7-ba42-1fee48abcbaf" xlink:to="loc_fstr_SteelProductsMember_7e3117e5-5667-4678-bf73-269e6cf447c1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_6e4d3aff-ac16-4c74-9875-6eec47724430" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_1edc6bc0-f121-4633-a474-1397ccbfa91d" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_6e4d3aff-ac16-4c74-9875-6eec47724430" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_6e4d3aff-ac16-4c74-9875-6eec47724430_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_6e4d3aff-ac16-4c74-9875-6eec47724430" xlink:to="loc_us-gaap_SegmentDomain_6e4d3aff-ac16-4c74-9875-6eec47724430_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_fe663010-6ebe-4cda-b817-59434cce5731" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_6e4d3aff-ac16-4c74-9875-6eec47724430" xlink:to="loc_us-gaap_SegmentDomain_fe663010-6ebe-4cda-b817-59434cce5731" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesAndServicesSegmentMember_3027b6b7-0ba8-4ce4-aee4-581955b02777" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesAndServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_fe663010-6ebe-4cda-b817-59434cce5731" xlink:to="loc_fstr_RailTechnologiesAndServicesSegmentMember_3027b6b7-0ba8-4ce4-aee4-581955b02777" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_81a0e4ae-6661-48ae-ba95-26756ac95483" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_fe663010-6ebe-4cda-b817-59434cce5731" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_81a0e4ae-6661-48ae-ba95-26756ac95483" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_b4c478e0-4063-4e29-80cb-80e10eface83" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_f7c07cb8-91b8-46fe-9e6e-386504886d1e" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_b4c478e0-4063-4e29-80cb-80e10eface83" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RevenueNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RevenueNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RevenueNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_e8b17cfe-709c-42ef-8b45-60a935e859df" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_d63fb272-967e-4dff-82ec-fd018a064937" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_e8b17cfe-709c-42ef-8b45-60a935e859df" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_d63fb272-967e-4dff-82ec-fd018a064937" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_1ff024d5-59b3-4d36-8e7d-e6fb64180d97" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_d63fb272-967e-4dff-82ec-fd018a064937" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_1ff024d5-59b3-4d36-8e7d-e6fb64180d97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_1ff024d5-59b3-4d36-8e7d-e6fb64180d97_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_1ff024d5-59b3-4d36-8e7d-e6fb64180d97" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_1ff024d5-59b3-4d36-8e7d-e6fb64180d97_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_88ee2a87-5293-4262-80e5-9f7d5e2041fa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_1ff024d5-59b3-4d36-8e7d-e6fb64180d97" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_88ee2a87-5293-4262-80e5-9f7d5e2041fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_692cce04-2e83-4897-b9fe-487074b20fa5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_88ee2a87-5293-4262-80e5-9f7d5e2041fa" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_692cce04-2e83-4897-b9fe-487074b20fa5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_967b2e80-b4cc-4165-86c7-f99a60855a35" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_d63fb272-967e-4dff-82ec-fd018a064937" xlink:to="loc_srt_ProductOrServiceAxis_967b2e80-b4cc-4165-86c7-f99a60855a35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_967b2e80-b4cc-4165-86c7-f99a60855a35_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_967b2e80-b4cc-4165-86c7-f99a60855a35" xlink:to="loc_srt_ProductsAndServicesDomain_967b2e80-b4cc-4165-86c7-f99a60855a35_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_b413db1d-3452-4988-960a-e11654034610" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_967b2e80-b4cc-4165-86c7-f99a60855a35" xlink:to="loc_srt_ProductsAndServicesDomain_b413db1d-3452-4988-960a-e11654034610" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LongTermCommercialContractsMember_8ab51e13-29ed-41c7-91d7-92582eae2f0a" xlink:href="fstr-20241231.xsd#fstr_LongTermCommercialContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_b413db1d-3452-4988-960a-e11654034610" xlink:to="loc_fstr_LongTermCommercialContractsMember_8ab51e13-29ed-41c7-91d7-92582eae2f0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_27d3f85e-06f7-46c1-8b03-b3e4060e1515" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ChangeInAccountingEstimateByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_d63fb272-967e-4dff-82ec-fd018a064937" xlink:to="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_27d3f85e-06f7-46c1-8b03-b3e4060e1515" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_27d3f85e-06f7-46c1-8b03-b3e4060e1515_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ChangeInAccountingEstimateTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_27d3f85e-06f7-46c1-8b03-b3e4060e1515" xlink:to="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_27d3f85e-06f7-46c1-8b03-b3e4060e1515_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_b75d6fe4-3ffc-4baa-9d84-d14a19e947c6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ChangeInAccountingEstimateTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_27d3f85e-06f7-46c1-8b03-b3e4060e1515" xlink:to="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_b75d6fe4-3ffc-4baa-9d84-d14a19e947c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesReturnsAndAllowancesMember_42997a19-8008-4bf7-9a01-5bccb425c697" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SalesReturnsAndAllowancesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_b75d6fe4-3ffc-4baa-9d84-d14a19e947c6" xlink:to="loc_us-gaap_SalesReturnsAndAllowancesMember_42997a19-8008-4bf7-9a01-5bccb425c697" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_a608a45a-1f28-42e9-bd07-53c249454146" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_e8b17cfe-709c-42ef-8b45-60a935e859df" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_a608a45a-1f28-42e9-bd07-53c249454146" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RevenueFromContractWithCustomerPercentage_0e1aa662-a3e8-4b30-a324-b67858c9eed4" xlink:href="fstr-20241231.xsd#fstr_RevenueFromContractWithCustomerPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_e8b17cfe-709c-42ef-8b45-60a935e859df" xlink:to="loc_fstr_RevenueFromContractWithCustomerPercentage_0e1aa662-a3e8-4b30-a324-b67858c9eed4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_76bc345b-5498-4e29-98e3-29d2d6e51d0b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_e8b17cfe-709c-42ef-8b45-60a935e859df" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_76bc345b-5498-4e29-98e3-29d2d6e51d0b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_7ff99778-4a4f-4b63-a874-3454b8692534" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_e8b17cfe-709c-42ef-8b45-60a935e859df" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_7ff99778-4a4f-4b63-a874-3454b8692534" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RevenueScheduleofOverTimeSalesDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RevenueScheduleofOverTimeSalesDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RevenueScheduleofOverTimeSalesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_1ee0d414-3f07-422e-a6ce-a4064b0a27d9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_1e43a9bb-6515-4e52-98e3-a649a7719792" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1ee0d414-3f07-422e-a6ce-a4064b0a27d9" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_1e43a9bb-6515-4e52-98e3-a649a7719792" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_873116a2-ab36-4a9a-9b4c-9073fbd79f3a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_1e43a9bb-6515-4e52-98e3-a649a7719792" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_873116a2-ab36-4a9a-9b4c-9073fbd79f3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_873116a2-ab36-4a9a-9b4c-9073fbd79f3a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_873116a2-ab36-4a9a-9b4c-9073fbd79f3a" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_873116a2-ab36-4a9a-9b4c-9073fbd79f3a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_6c81fc16-0a1d-426b-a540-dac6a563143b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_873116a2-ab36-4a9a-9b4c-9073fbd79f3a" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_6c81fc16-0a1d-426b-a540-dac6a563143b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OverTimeInputMethodMember_52bc0a8d-34cb-4ec2-a139-d78c73a7e2e1" xlink:href="fstr-20241231.xsd#fstr_OverTimeInputMethodMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_6c81fc16-0a1d-426b-a540-dac6a563143b" xlink:to="loc_fstr_OverTimeInputMethodMember_52bc0a8d-34cb-4ec2-a139-d78c73a7e2e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OverTimeOutputMethodMember_82d99f61-2afb-494c-a3d8-878d0a4891a4" xlink:href="fstr-20241231.xsd#fstr_OverTimeOutputMethodMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_6c81fc16-0a1d-426b-a540-dac6a563143b" xlink:to="loc_fstr_OverTimeOutputMethodMember_82d99f61-2afb-494c-a3d8-878d0a4891a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OverTimeSalesAmount_48232742-167a-4f6a-ac57-d36758a003c4" xlink:href="fstr-20241231.xsd#fstr_OverTimeSalesAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1ee0d414-3f07-422e-a6ce-a4064b0a27d9" xlink:to="loc_fstr_OverTimeSalesAmount_48232742-167a-4f6a-ac57-d36758a003c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OverTimeSalesPercent_c11a5fd3-ba80-434a-9c2a-1a5d214bf5f2" xlink:href="fstr-20241231.xsd#fstr_OverTimeSalesPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1ee0d414-3f07-422e-a6ce-a4064b0a27d9" xlink:to="loc_fstr_OverTimeSalesPercent_c11a5fd3-ba80-434a-9c2a-1a5d214bf5f2" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RevenueScheduleofTimingofTransferDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RevenueScheduleofTimingofTransferDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RevenueScheduleofTimingofTransferDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_7d7d9b0a-4b42-4d5f-92e7-e6d1dbeefad3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_8d3e5f46-1e49-4c67-bf55-cc9bc2ef60e4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7d7d9b0a-4b42-4d5f-92e7-e6d1dbeefad3" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_8d3e5f46-1e49-4c67-bf55-cc9bc2ef60e4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_5b1db71a-7d7e-4fff-87ed-2482a5751e5c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_8d3e5f46-1e49-4c67-bf55-cc9bc2ef60e4" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_5b1db71a-7d7e-4fff-87ed-2482a5751e5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_5b1db71a-7d7e-4fff-87ed-2482a5751e5c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_5b1db71a-7d7e-4fff-87ed-2482a5751e5c" xlink:to="loc_us-gaap_SegmentDomain_5b1db71a-7d7e-4fff-87ed-2482a5751e5c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_be86f5f0-5e48-404a-adab-8151f07e698b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_5b1db71a-7d7e-4fff-87ed-2482a5751e5c" xlink:to="loc_us-gaap_SegmentDomain_be86f5f0-5e48-404a-adab-8151f07e698b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesAndServicesSegmentMember_e38ea13a-2bdd-4063-a41a-6ae02483b6b5" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesAndServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_be86f5f0-5e48-404a-adab-8151f07e698b" xlink:to="loc_fstr_RailTechnologiesAndServicesSegmentMember_e38ea13a-2bdd-4063-a41a-6ae02483b6b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_e9687904-f12e-4c44-ae3c-afdea2369d96" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_be86f5f0-5e48-404a-adab-8151f07e698b" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_e9687904-f12e-4c44-ae3c-afdea2369d96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_027625e8-4296-4f8f-83db-d1e8355618a2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_8d3e5f46-1e49-4c67-bf55-cc9bc2ef60e4" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_027625e8-4296-4f8f-83db-d1e8355618a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_027625e8-4296-4f8f-83db-d1e8355618a2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_027625e8-4296-4f8f-83db-d1e8355618a2" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_027625e8-4296-4f8f-83db-d1e8355618a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_ae42d344-60ef-4fc6-8eb7-3105ed836e9e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_027625e8-4296-4f8f-83db-d1e8355618a2" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_ae42d344-60ef-4fc6-8eb7-3105ed836e9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_a017a3c7-e0ec-46f6-a2d4-5ca5a758f1bc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_ae42d344-60ef-4fc6-8eb7-3105ed836e9e" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_a017a3c7-e0ec-46f6-a2d4-5ca5a758f1bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_dbb44b74-e6b9-44f0-bdf8-1175558d1e77" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_ae42d344-60ef-4fc6-8eb7-3105ed836e9e" xlink:to="loc_us-gaap_TransferredOverTimeMember_dbb44b74-e6b9-44f0-bdf8-1175558d1e77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_20d5f3c2-24d4-44e4-95a2-37d8587235c5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7d7d9b0a-4b42-4d5f-92e7-e6d1dbeefad3" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_20d5f3c2-24d4-44e4-95a2-37d8587235c5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RevenueRemainingPerformanceObligationDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RevenueRemainingPerformanceObligationDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RevenueRemainingPerformanceObligationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_00de1cdd-73e1-490f-ac97-4cd158699950" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_00de1cdd-73e1-490f-ac97-4cd158699950" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_7730b6a5-7e3b-4aa6-926e-8aab7f8f2d67" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_00de1cdd-73e1-490f-ac97-4cd158699950" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_7730b6a5-7e3b-4aa6-926e-8aab7f8f2d67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_e54398d6-e3cc-47a1-b6a8-90d45d36ef75" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_e54398d6-e3cc-47a1-b6a8-90d45d36ef75" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6b8f8397-168f-4b92-8e9a-cf54d14c022e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6b8f8397-168f-4b92-8e9a-cf54d14c022e" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RevenueRemainingPerformanceObligationDetails_1" xlink:type="simple" xlink:href="fstr-20241231.xsd#RevenueRemainingPerformanceObligationDetails_1"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RevenueRemainingPerformanceObligationDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_00de1cdd-73e1-490f-ac97-4cd158699950" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_00de1cdd-73e1-490f-ac97-4cd158699950" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_e54398d6-e3cc-47a1-b6a8-90d45d36ef75" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_e54398d6-e3cc-47a1-b6a8-90d45d36ef75" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6b8f8397-168f-4b92-8e9a-cf54d14c022e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_b455cb7d-5211-4342-9ce5-f41f145006ee" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_6b8f8397-168f-4b92-8e9a-cf54d14c022e" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/GoodwillandOtherIntangibleAssetsScheduleofGoodwillDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#GoodwillandOtherIntangibleAssetsScheduleofGoodwillDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/GoodwillandOtherIntangibleAssetsScheduleofGoodwillDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_476101e1-510a-40ff-b3d6-469af7cd01ed" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_68b4624d-30ae-44c7-845c-c4273bd5d0dd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_476101e1-510a-40ff-b3d6-469af7cd01ed" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_68b4624d-30ae-44c7-845c-c4273bd5d0dd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_d207ac3a-c364-43f1-be43-e6defec964a7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_68b4624d-30ae-44c7-845c-c4273bd5d0dd" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_d207ac3a-c364-43f1-be43-e6defec964a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_d207ac3a-c364-43f1-be43-e6defec964a7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_d207ac3a-c364-43f1-be43-e6defec964a7" xlink:to="loc_us-gaap_SegmentDomain_d207ac3a-c364-43f1-be43-e6defec964a7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_dee999e7-4439-431b-b382-752ccc756c86" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_d207ac3a-c364-43f1-be43-e6defec964a7" xlink:to="loc_us-gaap_SegmentDomain_dee999e7-4439-431b-b382-752ccc756c86" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesAndServicesSegmentMember_74ca129a-eb23-4354-bf8b-df544bd864da" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesAndServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_dee999e7-4439-431b-b382-752ccc756c86" xlink:to="loc_fstr_RailTechnologiesAndServicesSegmentMember_74ca129a-eb23-4354-bf8b-df544bd864da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_f63c679e-cf20-469c-9b86-0ccb60f988eb" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_dee999e7-4439-431b-b382-752ccc756c86" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_f63c679e-cf20-469c-9b86-0ccb60f988eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_231b3a71-20d2-4391-b4be-8a2b921df7c5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_476101e1-510a-40ff-b3d6-469af7cd01ed" xlink:to="loc_us-gaap_GoodwillRollForward_231b3a71-20d2-4391-b4be-8a2b921df7c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_5c13f9b1-1c5c-4e95-8447-e8a16ff366b4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_231b3a71-20d2-4391-b4be-8a2b921df7c5" xlink:to="loc_us-gaap_Goodwill_5c13f9b1-1c5c-4e95-8447-e8a16ff366b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_9d643c04-c77e-45ca-9981-f0869da26be5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_231b3a71-20d2-4391-b4be-8a2b921df7c5" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_9d643c04-c77e-45ca-9981-f0869da26be5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_5df5c42c-02b8-4cf4-b0b8-205a27b6da24" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_231b3a71-20d2-4391-b4be-8a2b921df7c5" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_5df5c42c-02b8-4cf4-b0b8-205a27b6da24" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_4e4157fd-1295-4584-bb54-a1889f5aebb7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillForeignCurrencyTranslationGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_231b3a71-20d2-4391-b4be-8a2b921df7c5" xlink:to="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_4e4157fd-1295-4584-bb54-a1889f5aebb7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_a627b1c1-a1da-47e1-ac29-f7b36d28e76a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/GoodwillandOtherIntangibleAssetsNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#GoodwillandOtherIntangibleAssetsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/GoodwillandOtherIntangibleAssetsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_a88c4a97-d031-471c-8f4a-4eceb57bd0b3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_a88c4a97-d031-471c-8f4a-4eceb57bd0b3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_c31594f2-30a1-4115-9cce-858e6a72e6e3" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_a88c4a97-d031-471c-8f4a-4eceb57bd0b3" xlink:to="loc_srt_RangeAxis_c31594f2-30a1-4115-9cce-858e6a72e6e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c31594f2-30a1-4115-9cce-858e6a72e6e3_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_c31594f2-30a1-4115-9cce-858e6a72e6e3" xlink:to="loc_srt_RangeMember_c31594f2-30a1-4115-9cce-858e6a72e6e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_5d1c97dc-b66c-4c39-a339-573e39860d8b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_c31594f2-30a1-4115-9cce-858e6a72e6e3" xlink:to="loc_srt_RangeMember_5d1c97dc-b66c-4c39-a339-573e39860d8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_96811c07-83dc-4563-b5a3-9c1b976d408f" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_5d1c97dc-b66c-4c39-a339-573e39860d8b" xlink:to="loc_srt_WeightedAverageMember_96811c07-83dc-4563-b5a3-9c1b976d408f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_bfc65bf7-d638-4b9b-bdd2-6e91d243d9d3" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_5d1c97dc-b66c-4c39-a339-573e39860d8b" xlink:to="loc_srt_MinimumMember_bfc65bf7-d638-4b9b-bdd2-6e91d243d9d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_083ac100-1eb9-44d2-8393-bea251772084" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_5d1c97dc-b66c-4c39-a339-573e39860d8b" xlink:to="loc_srt_MaximumMember_083ac100-1eb9-44d2-8393-bea251772084" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_05966228-564b-4176-8979-90f12b412bfe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_a88c4a97-d031-471c-8f4a-4eceb57bd0b3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_05966228-564b-4176-8979-90f12b412bfe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_05966228-564b-4176-8979-90f12b412bfe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_05966228-564b-4176-8979-90f12b412bfe" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_05966228-564b-4176-8979-90f12b412bfe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6652a519-bf4f-4a3f-8af7-6c5d1711751c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_05966228-564b-4176-8979-90f12b412bfe" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6652a519-bf4f-4a3f-8af7-6c5d1711751c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_528287ae-68b7-4207-ba32-077229c604cf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6652a519-bf4f-4a3f-8af7-6c5d1711751c" xlink:to="loc_us-gaap_CustomerRelationshipsMember_528287ae-68b7-4207-ba32-077229c604cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_f2a78d0d-e5f5-4fc6-9cd3-1dd144ac734a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6652a519-bf4f-4a3f-8af7-6c5d1711751c" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_f2a78d0d-e5f5-4fc6-9cd3-1dd144ac734a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsPurchaseAccountingAdjustments_f0ee74bc-aec0-4141-ad88-0f2938b215b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsPurchaseAccountingAdjustments_f0ee74bc-aec0-4141-ad88-0f2938b215b1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_16cf0e46-61c9-43c6-a563-11cb1da8f333" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_16cf0e46-61c9-43c6-a563-11cb1da8f333" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_fd462982-928a-420b-ad45-f2e050fdab33" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_fd462982-928a-420b-ad45-f2e050fdab33" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_25d96610-8da2-4df4-a863-b94f7416668f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_25d96610-8da2-4df4-a863-b94f7416668f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_e31cc880-4a44-4d5d-838b-fb8272658362" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_e31cc880-4a44-4d5d-838b-fb8272658362" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_e1391a0d-aa57-401c-b1dc-be92bd3889dd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_838d453a-55d1-49f8-9a7f-bcb56a58311c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_e1391a0d-aa57-401c-b1dc-be92bd3889dd" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b7341fba-0e73-4805-8bd7-ec8f811303ed" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b6229d39-ec76-4511-9afb-f329c46b5cda" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b7341fba-0e73-4805-8bd7-ec8f811303ed" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b6229d39-ec76-4511-9afb-f329c46b5cda" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_4630d503-1390-4388-8ef4-20f2f0403c05" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b6229d39-ec76-4511-9afb-f329c46b5cda" xlink:to="loc_srt_RangeAxis_4630d503-1390-4388-8ef4-20f2f0403c05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_4630d503-1390-4388-8ef4-20f2f0403c05_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_4630d503-1390-4388-8ef4-20f2f0403c05" xlink:to="loc_srt_RangeMember_4630d503-1390-4388-8ef4-20f2f0403c05_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_7a99adf4-d517-4eef-8487-852f6256589d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_4630d503-1390-4388-8ef4-20f2f0403c05" xlink:to="loc_srt_RangeMember_7a99adf4-d517-4eef-8487-852f6256589d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_f6a92450-38ee-4ffb-a832-c37c793fe33f" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_7a99adf4-d517-4eef-8487-852f6256589d" xlink:to="loc_srt_WeightedAverageMember_f6a92450-38ee-4ffb-a832-c37c793fe33f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d567d3d7-96ac-4a91-ba77-07431277ce18" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b6229d39-ec76-4511-9afb-f329c46b5cda" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d567d3d7-96ac-4a91-ba77-07431277ce18" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d567d3d7-96ac-4a91-ba77-07431277ce18_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d567d3d7-96ac-4a91-ba77-07431277ce18" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d567d3d7-96ac-4a91-ba77-07431277ce18_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_09224a08-2463-452e-ab35-43df2ba8ec3c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d567d3d7-96ac-4a91-ba77-07431277ce18" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_09224a08-2463-452e-ab35-43df2ba8ec3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PatentsMember_508856ee-273e-4238-aa4d-2ed4edcad05f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PatentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_09224a08-2463-452e-ab35-43df2ba8ec3c" xlink:to="loc_us-gaap_PatentsMember_508856ee-273e-4238-aa4d-2ed4edcad05f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_381dcfad-3ba2-4b64-9ccb-5ca718daaad2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_09224a08-2463-452e-ab35-43df2ba8ec3c" xlink:to="loc_us-gaap_CustomerRelationshipsMember_381dcfad-3ba2-4b64-9ccb-5ca718daaad2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksAndTradeNamesMember_bc900422-70d0-47f1-a9ac-7cc9e052f7ac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TrademarksAndTradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_09224a08-2463-452e-ab35-43df2ba8ec3c" xlink:to="loc_us-gaap_TrademarksAndTradeNamesMember_bc900422-70d0-47f1-a9ac-7cc9e052f7ac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_e0520840-659c-418e-914f-cffb05fd48be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_09224a08-2463-452e-ab35-43df2ba8ec3c" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_e0520840-659c-418e-914f-cffb05fd48be" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OffMarketFavorableLeaseMember_a4483f12-c33d-4caa-8d9d-2e2c3ad3e2e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OffMarketFavorableLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_09224a08-2463-452e-ab35-43df2ba8ec3c" xlink:to="loc_us-gaap_OffMarketFavorableLeaseMember_a4483f12-c33d-4caa-8d9d-2e2c3ad3e2e6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_065c1f2d-6a2a-42eb-8852-5f9c58337b71" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b7341fba-0e73-4805-8bd7-ec8f811303ed" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_065c1f2d-6a2a-42eb-8852-5f9c58337b71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_6dc4e049-47a8-436d-97ee-23b0106302af" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b7341fba-0e73-4805-8bd7-ec8f811303ed" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_6dc4e049-47a8-436d-97ee-23b0106302af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_ed83cd69-07b0-4962-9bf5-83e086b9fae0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b7341fba-0e73-4805-8bd7-ec8f811303ed" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_ed83cd69-07b0-4962-9bf5-83e086b9fae0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_4ed12e63-ceb3-4774-aa01-ae4a4b3fbaf1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b7341fba-0e73-4805-8bd7-ec8f811303ed" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_4ed12e63-ceb3-4774-aa01-ae4a4b3fbaf1" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/AccountsReceivableNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#AccountsReceivableNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/AccountsReceivableNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_ede3e349-22f1-4fb9-8b8c-efefd440db32" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_18a8bb34-925e-4ff3-901c-7fd0520fb41f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_ede3e349-22f1-4fb9-8b8c-efefd440db32" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_18a8bb34-925e-4ff3-901c-7fd0520fb41f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_cc5b1cf5-f94e-4dae-859d-bf480638d98b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_18a8bb34-925e-4ff3-901c-7fd0520fb41f" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_cc5b1cf5-f94e-4dae-859d-bf480638d98b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_cc5b1cf5-f94e-4dae-859d-bf480638d98b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_cc5b1cf5-f94e-4dae-859d-bf480638d98b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_cc5b1cf5-f94e-4dae-859d-bf480638d98b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_ce489112-07c4-4a0b-becc-54b910a6596d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_cc5b1cf5-f94e-4dae-859d-bf480638d98b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_ce489112-07c4-4a0b-becc-54b910a6596d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d828b568-cb10-478b-be1e-0a5e08ca478e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_ce489112-07c4-4a0b-becc-54b910a6596d" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d828b568-cb10-478b-be1e-0a5e08ca478e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_f99fa7de-2424-4bba-bd2d-2f5ac3178127" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProvisionForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_ede3e349-22f1-4fb9-8b8c-efefd440db32" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_f99fa7de-2424-4bba-bd2d-2f5ac3178127" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries_5b41fff6-20ae-4a76-b80a-03780eb68ef0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_ede3e349-22f1-4fb9-8b8c-efefd440db32" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries_5b41fff6-20ae-4a76-b80a-03780eb68ef0" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/PropertyPlantandEquipmentDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#PropertyPlantandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/PropertyPlantandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6c1e714d-8287-468e-a2e2-c23dbcc73267" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_d55c4c86-4a20-444f-8f95-721db47aa55f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6c1e714d-8287-468e-a2e2-c23dbcc73267" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_d55c4c86-4a20-444f-8f95-721db47aa55f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_ec1d2220-b853-46da-bf8b-c734d0a90942" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_d55c4c86-4a20-444f-8f95-721db47aa55f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_ec1d2220-b853-46da-bf8b-c734d0a90942" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_ec1d2220-b853-46da-bf8b-c734d0a90942_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_ec1d2220-b853-46da-bf8b-c734d0a90942" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_ec1d2220-b853-46da-bf8b-c734d0a90942_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f0dd0ee0-eced-48d7-b15a-24448ac95a31" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_ec1d2220-b853-46da-bf8b-c734d0a90942" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f0dd0ee0-eced-48d7-b15a-24448ac95a31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandMember_44deb12e-2bc8-4659-83c2-605351e472d6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f0dd0ee0-eced-48d7-b15a-24448ac95a31" xlink:to="loc_us-gaap_LandMember_44deb12e-2bc8-4659-83c2-605351e472d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_ImprovementsToLandAndLeaseholdsMember_91fd115d-cade-4083-a974-f8cff9fad728" xlink:href="fstr-20241231.xsd#fstr_ImprovementsToLandAndLeaseholdsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f0dd0ee0-eced-48d7-b15a-24448ac95a31" xlink:to="loc_fstr_ImprovementsToLandAndLeaseholdsMember_91fd115d-cade-4083-a974-f8cff9fad728" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingMember_d03c74bf-1db4-4eaa-b583-03a30d7297ff" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f0dd0ee0-eced-48d7-b15a-24448ac95a31" xlink:to="loc_us-gaap_BuildingMember_d03c74bf-1db4-4eaa-b583-03a30d7297ff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_MachineryAndEquipmentIncludingEquipmentUnderCapitalizedLeasesMember_a2100872-63c8-418e-a5a7-beed9d861ead" xlink:href="fstr-20241231.xsd#fstr_MachineryAndEquipmentIncludingEquipmentUnderCapitalizedLeasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f0dd0ee0-eced-48d7-b15a-24448ac95a31" xlink:to="loc_fstr_MachineryAndEquipmentIncludingEquipmentUnderCapitalizedLeasesMember_a2100872-63c8-418e-a5a7-beed9d861ead" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_4a1bf5e8-828a-4ecf-b275-c2f577e45f9e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f0dd0ee0-eced-48d7-b15a-24448ac95a31" xlink:to="loc_us-gaap_ConstructionInProgressMember_4a1bf5e8-828a-4ecf-b275-c2f577e45f9e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_12b63fd5-9bca-4252-984e-100072b28f19" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6c1e714d-8287-468e-a2e2-c23dbcc73267" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_12b63fd5-9bca-4252-984e-100072b28f19" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_0c36ef28-bbba-4e97-8b95-a3b184b1ea15" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6c1e714d-8287-468e-a2e2-c23dbcc73267" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_0c36ef28-bbba-4e97-8b95-a3b184b1ea15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_f60c4b6c-3e7f-40b2-96db-0fe421813b06" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6c1e714d-8287-468e-a2e2-c23dbcc73267" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_f60c4b6c-3e7f-40b2-96db-0fe421813b06" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_f2fa8616-ce7b-4c07-a44e-0e920ec86eae" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6c1e714d-8287-468e-a2e2-c23dbcc73267" xlink:to="loc_us-gaap_Depreciation_f2fa8616-ce7b-4c07-a44e-0e920ec86eae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_31b1b517-0087-49e0-b42e-d114fe0d2074" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6c1e714d-8287-468e-a2e2-c23dbcc73267" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_31b1b517-0087-49e0-b42e-d114fe0d2074" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/LeasesNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#LeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/LeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_a793fe05-c321-4b7f-be07-f1dd8382e43b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_90a14636-e541-4e8a-ba87-7cf03799eadd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a793fe05-c321-4b7f-be07-f1dd8382e43b" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_90a14636-e541-4e8a-ba87-7cf03799eadd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_a76a500c-8dbf-496e-840e-aa4778c2d167" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_90a14636-e541-4e8a-ba87-7cf03799eadd" xlink:to="loc_srt_RangeAxis_a76a500c-8dbf-496e-840e-aa4778c2d167" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a76a500c-8dbf-496e-840e-aa4778c2d167_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_a76a500c-8dbf-496e-840e-aa4778c2d167" xlink:to="loc_srt_RangeMember_a76a500c-8dbf-496e-840e-aa4778c2d167_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_323265d9-0d25-44a5-8856-c151c89c5265" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_a76a500c-8dbf-496e-840e-aa4778c2d167" xlink:to="loc_srt_RangeMember_323265d9-0d25-44a5-8856-c151c89c5265" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_925facf9-a071-41e6-a503-7c954fbbf967" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_323265d9-0d25-44a5-8856-c151c89c5265" xlink:to="loc_srt_MinimumMember_925facf9-a071-41e6-a503-7c954fbbf967" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_579dcc00-1194-4747-868f-eec17b0ad4e1" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_323265d9-0d25-44a5-8856-c151c89c5265" xlink:to="loc_srt_MaximumMember_579dcc00-1194-4747-868f-eec17b0ad4e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LesseeRemainingLeaseTerm_fe36acf2-202f-4658-a1a0-d7b5cc169a14" xlink:href="fstr-20241231.xsd#fstr_LesseeRemainingLeaseTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a793fe05-c321-4b7f-be07-f1dd8382e43b" xlink:to="loc_fstr_LesseeRemainingLeaseTerm_fe36acf2-202f-4658-a1a0-d7b5cc169a14" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LesseeLeaseRenewalTerm_983a32e9-be20-46c5-a409-1167ca68e09f" xlink:href="fstr-20241231.xsd#fstr_LesseeLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a793fe05-c321-4b7f-be07-f1dd8382e43b" xlink:to="loc_fstr_LesseeLeaseRenewalTerm_983a32e9-be20-46c5-a409-1167ca68e09f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LesseeLeaseTerminationPeriod_08fd2b97-705b-45c7-8baf-5f0bc0255af9" xlink:href="fstr-20241231.xsd#fstr_LesseeLeaseTerminationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a793fe05-c321-4b7f-be07-f1dd8382e43b" xlink:to="loc_fstr_LesseeLeaseTerminationPeriod_08fd2b97-705b-45c7-8baf-5f0bc0255af9" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/LongTermDebtandRelatedMattersNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#LongTermDebtandRelatedMattersNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/LongTermDebtandRelatedMattersNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_dfd29f6b-03de-4243-a476-d3a4a2dd9085" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_dfd29f6b-03de-4243-a476-d3a4a2dd9085" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_e8ddd489-4a37-4138-ad93-204d01bc0bf0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_dfd29f6b-03de-4243-a476-d3a4a2dd9085" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_e8ddd489-4a37-4138-ad93-204d01bc0bf0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_e8ddd489-4a37-4138-ad93-204d01bc0bf0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_e8ddd489-4a37-4138-ad93-204d01bc0bf0" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_e8ddd489-4a37-4138-ad93-204d01bc0bf0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_458288fa-9017-4e6b-86ba-a82a95b2951b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_e8ddd489-4a37-4138-ad93-204d01bc0bf0" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_458288fa-9017-4e6b-86ba-a82a95b2951b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_PNCBankNACitizensBankNAWellsFargoBankNationalAssociationBankOfAmericaNAAndBMOHarrisBankNationalAssociationMember_40c96da6-f9c1-4d9a-b412-261be16d8b2e" xlink:href="fstr-20241231.xsd#fstr_PNCBankNACitizensBankNAWellsFargoBankNationalAssociationBankOfAmericaNAAndBMOHarrisBankNationalAssociationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_458288fa-9017-4e6b-86ba-a82a95b2951b" xlink:to="loc_fstr_PNCBankNACitizensBankNAWellsFargoBankNationalAssociationBankOfAmericaNAAndBMOHarrisBankNationalAssociationMember_40c96da6-f9c1-4d9a-b412-261be16d8b2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_PncBankN.a.BankOfAmericaN.a.WellsFargoBankN.a.CitizensBankOfPennsylvaniaAndBranchBankingAndTrustCompanyMember_ed90a517-0042-43d1-939b-2025a71890ab" xlink:href="fstr-20241231.xsd#fstr_PncBankN.a.BankOfAmericaN.a.WellsFargoBankN.a.CitizensBankOfPennsylvaniaAndBranchBankingAndTrustCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_458288fa-9017-4e6b-86ba-a82a95b2951b" xlink:to="loc_fstr_PncBankN.a.BankOfAmericaN.a.WellsFargoBankN.a.CitizensBankOfPennsylvaniaAndBranchBankingAndTrustCompanyMember_ed90a517-0042-43d1-939b-2025a71890ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_357c4610-ade0-4d62-a1c6-6cb90fa3e389" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_dfd29f6b-03de-4243-a476-d3a4a2dd9085" xlink:to="loc_us-gaap_CreditFacilityAxis_357c4610-ade0-4d62-a1c6-6cb90fa3e389" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_357c4610-ade0-4d62-a1c6-6cb90fa3e389_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_357c4610-ade0-4d62-a1c6-6cb90fa3e389" xlink:to="loc_us-gaap_CreditFacilityDomain_357c4610-ade0-4d62-a1c6-6cb90fa3e389_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_fef19b23-47c1-4bd2-bffb-d80d31713edc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_357c4610-ade0-4d62-a1c6-6cb90fa3e389" xlink:to="loc_us-gaap_CreditFacilityDomain_fef19b23-47c1-4bd2-bffb-d80d31713edc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_2bf180e8-15c1-4418-9a35-7edcd6913e9f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_fef19b23-47c1-4bd2-bffb-d80d31713edc" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_2bf180e8-15c1-4418-9a35-7edcd6913e9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_2911e326-0930-467a-94af-468670891bb7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_dfd29f6b-03de-4243-a476-d3a4a2dd9085" xlink:to="loc_us-gaap_DebtInstrumentAxis_2911e326-0930-467a-94af-468670891bb7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2911e326-0930-467a-94af-468670891bb7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_2911e326-0930-467a-94af-468670891bb7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2911e326-0930-467a-94af-468670891bb7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_b1b299f2-7fbc-4549-bcd5-d8223759003a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_2911e326-0930-467a-94af-468670891bb7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_b1b299f2-7fbc-4549-bcd5-d8223759003a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_FourthAmendedAndRestatedCreditAgreementMember_851f488c-0b8b-4fd2-90c2-da36fb067906" xlink:href="fstr-20241231.xsd#fstr_FourthAmendedAndRestatedCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_b1b299f2-7fbc-4549-bcd5-d8223759003a" xlink:to="loc_fstr_FourthAmendedAndRestatedCreditAgreementMember_851f488c-0b8b-4fd2-90c2-da36fb067906" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_bf9d3c70-73ba-4a4c-997b-7ce392437a0f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentRedemptionPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_dfd29f6b-03de-4243-a476-d3a4a2dd9085" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_bf9d3c70-73ba-4a4c-997b-7ce392437a0f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_bf9d3c70-73ba-4a4c-997b-7ce392437a0f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_bf9d3c70-73ba-4a4c-997b-7ce392437a0f" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_bf9d3c70-73ba-4a4c-997b-7ce392437a0f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_92a9733e-fd6b-4a9d-b088-dad084d344a8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_bf9d3c70-73ba-4a4c-997b-7ce392437a0f" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_92a9733e-fd6b-4a9d-b088-dad084d344a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_d0c9e19f-c0f2-4983-b0c3-918ced8d2fa4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentRedemptionPeriodOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_92a9733e-fd6b-4a9d-b088-dad084d344a8" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_d0c9e19f-c0f2-4983-b0c3-918ced8d2fa4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodTwoMember_3b06b0d4-2bde-4d32-b0a9-74d619a0b8c7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentRedemptionPeriodTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_92a9733e-fd6b-4a9d-b088-dad084d344a8" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodTwoMember_3b06b0d4-2bde-4d32-b0a9-74d619a0b8c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_ee3b2e7e-0e8e-4a56-b79b-7ea173d2a39a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_us-gaap_DebtInstrumentTerm_ee3b2e7e-0e8e-4a56-b79b-7ea173d2a39a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_8399a41c-655d-4e96-8169-c0c2f42dc236" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_8399a41c-655d-4e96-8169-c0c2f42dc236" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LineOfCreditAccordionFeatureLowerBorrowingCapacityOption_6ff21e1b-a81d-414f-89d7-2d8c50fbdaa9" xlink:href="fstr-20241231.xsd#fstr_LineOfCreditAccordionFeatureLowerBorrowingCapacityOption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_fstr_LineOfCreditAccordionFeatureLowerBorrowingCapacityOption_6ff21e1b-a81d-414f-89d7-2d8c50fbdaa9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LineOfCreditFacilityMaximumIncreaseInCommitments_b46b67da-e4a5-40b2-b265-dafc84d538e0" xlink:href="fstr-20241231.xsd#fstr_LineOfCreditFacilityMaximumIncreaseInCommitments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_fstr_LineOfCreditFacilityMaximumIncreaseInCommitments_b46b67da-e4a5-40b2-b265-dafc84d538e0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_DebtInstrumentNumberOfFinancialCovenants_40da50bf-d01e-4c03-b56a-8aba4d625fb6" xlink:href="fstr-20241231.xsd#fstr_DebtInstrumentNumberOfFinancialCovenants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_fstr_DebtInstrumentNumberOfFinancialCovenants_40da50bf-d01e-4c03-b56a-8aba4d625fb6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_DebtInstrumentCovenantLeverageRatioMaximum_4d21fe41-b45b-47b2-a117-8a64e646d936" xlink:href="fstr-20241231.xsd#fstr_DebtInstrumentCovenantLeverageRatioMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_fstr_DebtInstrumentCovenantLeverageRatioMaximum_4d21fe41-b45b-47b2-a117-8a64e646d936" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_DebtInstrumentCovenantInterestCoverageRatioMinimum_71db9974-a152-4ee1-b0bd-1e6f03670a24" xlink:href="fstr-20241231.xsd#fstr_DebtInstrumentCovenantInterestCoverageRatioMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_fstr_DebtInstrumentCovenantInterestCoverageRatioMinimum_71db9974-a152-4ee1-b0bd-1e6f03670a24" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_dd39865d-a5f5-4701-a7b1-dd3c5eca46f2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_us-gaap_LineOfCredit_dd39865d-a5f5-4701-a7b1-dd3c5eca46f2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_9ecfa4be-1e7c-4d64-a1d6-fe6054249e99" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ba628e16-3608-46f9-a0c3-fc4e061c2cc1" xlink:to="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_9ecfa4be-1e7c-4d64-a1d6-fe6054249e99" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/StockholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#StockholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/StockholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_1e0a57fe-4594-4830-b366-702176d81981" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_1e0a57fe-4594-4830-b366-702176d81981" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_8002981e-2894-46b6-82e2-7fc88ab5c815" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_1e0a57fe-4594-4830-b366-702176d81981" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_8002981e-2894-46b6-82e2-7fc88ab5c815" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_8002981e-2894-46b6-82e2-7fc88ab5c815_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_8002981e-2894-46b6-82e2-7fc88ab5c815" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_8002981e-2894-46b6-82e2-7fc88ab5c815_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_073575a5-1906-4840-bb3f-7a90c2ed9d63" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_8002981e-2894-46b6-82e2-7fc88ab5c815" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_073575a5-1906-4840-bb3f-7a90c2ed9d63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_9410507f-596d-4609-83a3-c48c9eb6802d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_073575a5-1906-4840-bb3f-7a90c2ed9d63" xlink:to="loc_us-gaap_SubsequentEventMember_9410507f-596d-4609-83a3-c48c9eb6802d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_9951ef1f-f5e1-4010-a434-3024aef0e503" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_9951ef1f-f5e1-4010-a434-3024aef0e503" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_e56f6c9c-8e8c-4983-9fa9-a3e6d141d459" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_CommonStockSharesIssued_e56f6c9c-8e8c-4983-9fa9-a3e6d141d459" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_a9ddfbbb-c072-4449-9e72-29f27913f16d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_a9ddfbbb-c072-4449-9e72-29f27913f16d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_4e13ec7d-f30c-46eb-8b40-37f53be0535c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_4e13ec7d-f30c-46eb-8b40-37f53be0535c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_b3c58038-5cea-4db9-9e8a-fc6e9c96ba5f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_b3c58038-5cea-4db9-9e8a-fc6e9c96ba5f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_f627405e-cd88-49a7-8315-9806934c73f6" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_f627405e-cd88-49a7-8315-9806934c73f6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_StockRepurchaseProgramAuthorizedAmountAnnualLimitation_932f2d60-ccd6-4d0a-9514-e3a272bb9fa4" xlink:href="fstr-20241231.xsd#fstr_StockRepurchaseProgramAuthorizedAmountAnnualLimitation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_fstr_StockRepurchaseProgramAuthorizedAmountAnnualLimitation_932f2d60-ccd6-4d0a-9514-e3a272bb9fa4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_5934ca44-d4ae-4820-a842-ce90d45d8460" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_5934ca44-d4ae-4820-a842-ce90d45d8460" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_6615db41-e28f-4498-a682-8ca9acc5b5a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_6615db41-e28f-4498-a682-8ca9acc5b5a1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Dividends_5a317950-768f-4496-81a2-ad914ea8b49f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Dividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_fabe98b1-dfb2-4b9a-9246-d3b97f8492b7" xlink:to="loc_us-gaap_Dividends_5a317950-768f-4496-81a2-ad914ea8b49f" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/StockholdersEquityScheduleofRollforwardofCommonStockDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#StockholdersEquityScheduleofRollforwardofCommonStockDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/StockholdersEquityScheduleofRollforwardofCommonStockDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_d6ec2bec-88da-4512-a7fe-f6fa190165cd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_bf3d5e8c-935c-436f-b535-e67d64b4c49e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_d6ec2bec-88da-4512-a7fe-f6fa190165cd" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_bf3d5e8c-935c-436f-b535-e67d64b4c49e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_0c599b7e-b245-40f1-8959-ef6b5043a676" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_bf3d5e8c-935c-436f-b535-e67d64b4c49e" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_0c599b7e-b245-40f1-8959-ef6b5043a676" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_0c599b7e-b245-40f1-8959-ef6b5043a676_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_0c599b7e-b245-40f1-8959-ef6b5043a676" xlink:to="loc_us-gaap_EquityComponentDomain_0c599b7e-b245-40f1-8959-ef6b5043a676_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_d8f027c4-6393-4f94-af4f-1e2b356b9de8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_0c599b7e-b245-40f1-8959-ef6b5043a676" xlink:to="loc_us-gaap_EquityComponentDomain_d8f027c4-6393-4f94-af4f-1e2b356b9de8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_22cd5737-5ff6-4f7e-9d98-c141a8a2e507" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d8f027c4-6393-4f94-af4f-1e2b356b9de8" xlink:to="loc_us-gaap_TreasuryStockCommonMember_22cd5737-5ff6-4f7e-9d98-c141a8a2e507" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_1faf71d0-a0b8-4a2f-b9fe-6a31b965a389" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d8f027c4-6393-4f94-af4f-1e2b356b9de8" xlink:to="loc_us-gaap_CommonStockMember_1faf71d0-a0b8-4a2f-b9fe-6a31b965a389" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_d8f0dd98-868e-43f3-9bc0-39e8e76d3421" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d6ec2bec-88da-4512-a7fe-f6fa190165cd" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_d8f0dd98-868e-43f3-9bc0-39e8e76d3421" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_508f29e2-8407-45dc-a04a-dfa2e71d7165" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_d8f0dd98-868e-43f3-9bc0-39e8e76d3421" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_508f29e2-8407-45dc-a04a-dfa2e71d7165" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationForfeited_0b3dddcf-1ebe-4185-a1b3-f6381af5ced7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationForfeited"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_d8f0dd98-868e-43f3-9bc0-39e8e76d3421" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationForfeited_0b3dddcf-1ebe-4185-a1b3-f6381af5ced7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_5bb80434-89f2-4775-a460-6748a60f31bc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_d8f0dd98-868e-43f3-9bc0-39e8e76d3421" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_5bb80434-89f2-4775-a460-6748a60f31bc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_e0e5793c-86dd-4eee-afbd-47d9c2f7cc65" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_d8f0dd98-868e-43f3-9bc0-39e8e76d3421" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_e0e5793c-86dd-4eee-afbd-47d9c2f7cc65" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_866babe2-2f7f-4a5d-9806-f985e8047e8a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_d8f0dd98-868e-43f3-9bc0-39e8e76d3421" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_866babe2-2f7f-4a5d-9806-f985e8047e8a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_c784a2dc-766e-4287-a3f5-b1440267bca4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/AccumulatedOtherComprehensiveLossScheduleofAccumulatedOtherComprehensiveLossDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#AccumulatedOtherComprehensiveLossScheduleofAccumulatedOtherComprehensiveLossDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/AccumulatedOtherComprehensiveLossScheduleofAccumulatedOtherComprehensiveLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_21bda368-87c8-4ad5-b6c7-1fe35e37fdce" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_500aceb0-31a0-4a58-8cbb-c88d9e4f718e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_21bda368-87c8-4ad5-b6c7-1fe35e37fdce" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_500aceb0-31a0-4a58-8cbb-c88d9e4f718e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_fc4c8d3e-148a-483c-a7b7-d3dbb1eccc85" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_500aceb0-31a0-4a58-8cbb-c88d9e4f718e" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_fc4c8d3e-148a-483c-a7b7-d3dbb1eccc85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_fc4c8d3e-148a-483c-a7b7-d3dbb1eccc85_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_fc4c8d3e-148a-483c-a7b7-d3dbb1eccc85" xlink:to="loc_us-gaap_EquityComponentDomain_fc4c8d3e-148a-483c-a7b7-d3dbb1eccc85_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_c6b13d9b-4229-4aef-aa21-4241724678b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_fc4c8d3e-148a-483c-a7b7-d3dbb1eccc85" xlink:to="loc_us-gaap_EquityComponentDomain_c6b13d9b-4229-4aef-aa21-4241724678b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_d7d49d1e-cf3c-4f04-9919-9170de78892b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_c6b13d9b-4229-4aef-aa21-4241724678b1" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_d7d49d1e-cf3c-4f04-9919-9170de78892b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_3b64c7e9-61eb-4606-b86a-9f78efd8f031" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_d7d49d1e-cf3c-4f04-9919-9170de78892b" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_3b64c7e9-61eb-4606-b86a-9f78efd8f031" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_22888752-b00d-4473-b2f2-207029b409a3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_d7d49d1e-cf3c-4f04-9919-9170de78892b" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_22888752-b00d-4473-b2f2-207029b409a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_4bd17200-24d2-416c-9627-2d4026ab85a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_d7d49d1e-cf3c-4f04-9919-9170de78892b" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_4bd17200-24d2-416c-9627-2d4026ab85a1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_62aae378-906c-4899-bfc3-7e5ba3fcec96" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_21bda368-87c8-4ad5-b6c7-1fe35e37fdce" xlink:to="loc_us-gaap_StockholdersEquity_62aae378-906c-4899-bfc3-7e5ba3fcec96" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/AccumulatedOtherComprehensiveLossNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#AccumulatedOtherComprehensiveLossNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/AccumulatedOtherComprehensiveLossNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b79a4489-2b7e-48e1-8a21-bd503786ef40" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_2d1a3695-6e15-4e3c-a656-9ca1fb37a89d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b79a4489-2b7e-48e1-8a21-bd503786ef40" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_2d1a3695-6e15-4e3c-a656-9ca1fb37a89d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_261b52cc-6cf8-4433-ae95-0974c11acd40" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_2d1a3695-6e15-4e3c-a656-9ca1fb37a89d" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_261b52cc-6cf8-4433-ae95-0974c11acd40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_261b52cc-6cf8-4433-ae95-0974c11acd40_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_261b52cc-6cf8-4433-ae95-0974c11acd40" xlink:to="loc_us-gaap_EquityComponentDomain_261b52cc-6cf8-4433-ae95-0974c11acd40_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_c943251c-ebce-461e-b07b-5afcec38312d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_261b52cc-6cf8-4433-ae95-0974c11acd40" xlink:to="loc_us-gaap_EquityComponentDomain_c943251c-ebce-461e-b07b-5afcec38312d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_dea2e80b-362b-4954-af46-7e919d94da3f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_c943251c-ebce-461e-b07b-5afcec38312d" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_dea2e80b-362b-4954-af46-7e919d94da3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_3a388e64-feb5-4760-b879-edd725ef1877" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_2d1a3695-6e15-4e3c-a656-9ca1fb37a89d" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_3a388e64-feb5-4760-b879-edd725ef1877" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_3a388e64-feb5-4760-b879-edd725ef1877_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_3a388e64-feb5-4760-b879-edd725ef1877" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_3a388e64-feb5-4760-b879-edd725ef1877_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_07363905-5872-45d8-af15-46a02df5f282" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_3a388e64-feb5-4760-b879-edd725ef1877" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_07363905-5872-45d8-af15-46a02df5f282" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_ee266322-238d-4a99-8269-fccee0299da1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_07363905-5872-45d8-af15-46a02df5f282" xlink:to="loc_us-gaap_DomesticPlanMember_ee266322-238d-4a99-8269-fccee0299da1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_6f2a3af9-9fde-4592-9c68-52673ea22b1b" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_ee266322-238d-4a99-8269-fccee0299da1" xlink:to="loc_country_US_6f2a3af9-9fde-4592-9c68-52673ea22b1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_9e8eeece-cdba-410d-8df6-56f9d324809d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b79a4489-2b7e-48e1-8a21-bd503786ef40" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_9e8eeece-cdba-410d-8df6-56f9d324809d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_52e18783-1162-49f7-879d-be60b93301a7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_52e18783-1162-49f7-879d-be60b93301a7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_308cc753-ce06-46c6-aa17-9bd496e2eb45" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_52e18783-1162-49f7-879d-be60b93301a7" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_308cc753-ce06-46c6-aa17-9bd496e2eb45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_308cc753-ce06-46c6-aa17-9bd496e2eb45_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_308cc753-ce06-46c6-aa17-9bd496e2eb45" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_308cc753-ce06-46c6-aa17-9bd496e2eb45_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_dbb3ef2e-47df-418f-bcbc-94419a2055f2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_308cc753-ce06-46c6-aa17-9bd496e2eb45" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_dbb3ef2e-47df-418f-bcbc-94419a2055f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_622cec04-af4e-4833-8ece-fc7c4f403f04" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_dbb3ef2e-47df-418f-bcbc-94419a2055f2" xlink:to="loc_us-gaap_DomesticCountryMember_622cec04-af4e-4833-8ece-fc7c4f403f04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_50e44c07-9c90-4df8-858b-44667a147ee5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_dbb3ef2e-47df-418f-bcbc-94419a2055f2" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_50e44c07-9c90-4df8-858b-44667a147ee5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCountryMember_ca4fb90b-0a58-4ebf-82e3-80b067a8329f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_dbb3ef2e-47df-418f-bcbc-94419a2055f2" xlink:to="loc_us-gaap_ForeignCountryMember_ca4fb90b-0a58-4ebf-82e3-80b067a8329f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameAxis_5fd1f7d3-eb55-4adf-8940-2c812c8c0c82" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_52e18783-1162-49f7-879d-be60b93301a7" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameAxis_5fd1f7d3-eb55-4adf-8940-2c812c8c0c82" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_5fd1f7d3-eb55-4adf-8940-2c812c8c0c82_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_5fd1f7d3-eb55-4adf-8940-2c812c8c0c82" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_5fd1f7d3-eb55-4adf-8940-2c812c8c0c82_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_d81d7d12-5f52-4f22-9ead-922e90c25cc9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_5fd1f7d3-eb55-4adf-8940-2c812c8c0c82" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_d81d7d12-5f52-4f22-9ead-922e90c25cc9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecretariatOfTheFederalRevenueBureauOfBrazilMember_66f3bafd-0d2e-430c-b83d-30ed6e67a75b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SecretariatOfTheFederalRevenueBureauOfBrazilMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameDomain_d81d7d12-5f52-4f22-9ead-922e90c25cc9" xlink:to="loc_us-gaap_SecretariatOfTheFederalRevenueBureauOfBrazilMember_66f3bafd-0d2e-430c-b83d-30ed6e67a75b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_34040897-074e-43e7-8f32-6b71f8e9f628" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowanceByDeferredTaxAssetAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_52e18783-1162-49f7-879d-be60b93301a7" xlink:to="loc_us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_34040897-074e-43e7-8f32-6b71f8e9f628" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetDomain_34040897-074e-43e7-8f32-6b71f8e9f628_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredTaxAssetDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_34040897-074e-43e7-8f32-6b71f8e9f628" xlink:to="loc_us-gaap_DeferredTaxAssetDomain_34040897-074e-43e7-8f32-6b71f8e9f628_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetDomain_e8115e76-e1fd-4b55-a00e-94ff8f1ab361" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredTaxAssetDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_34040897-074e-43e7-8f32-6b71f8e9f628" xlink:to="loc_us-gaap_DeferredTaxAssetDomain_e8115e76-e1fd-4b55-a00e-94ff8f1ab361" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_StateOperatingLossCarryforwardMember_1b2f52b5-fc8e-4e9b-a470-17385f638aa4" xlink:href="fstr-20241231.xsd#fstr_StateOperatingLossCarryforwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredTaxAssetDomain_e8115e76-e1fd-4b55-a00e-94ff8f1ab361" xlink:to="loc_fstr_StateOperatingLossCarryforwardMember_1b2f52b5-fc8e-4e9b-a470-17385f638aa4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_8d427257-7a08-4f20-92f3-ac5163906e7f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_8d427257-7a08-4f20-92f3-ac5163906e7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_7c7d6f98-ef49-4d45-bb11-2f7ae72938f7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_OperatingLossCarryforwards_7c7d6f98-ef49-4d45-bb11-2f7ae72938f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsValuationAllowance_1b8a586d-5fc4-4abe-a1e5-25bbd8c05ad0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_DeferredTaxAssetsValuationAllowance_1b8a586d-5fc4-4abe-a1e5-25bbd8c05ad0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch_99ac1d1e-694e-4c34-abc1-b6555cd48a38" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch_99ac1d1e-694e-4c34-abc1-b6555cd48a38" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsValuationAllowance_e6123c08-e69b-4347-91ef-aa86fa7b430d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLossCarryforwardsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_OperatingLossCarryforwardsValuationAllowance_e6123c08-e69b-4347-91ef-aa86fa7b430d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings_406adf57-98d9-4adc-9c4b-06bc62e62efb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings_406adf57-98d9-4adc-9c4b-06bc62e62efb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignEarningsRepatriated_255bf9c9-355d-4d45-967c-de7c916f80dc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignEarningsRepatriated"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_ForeignEarningsRepatriated_255bf9c9-355d-4d45-967c-de7c916f80dc" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_a0f8fa0a-f0fb-4d53-abce-d39e92df5201" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_a0f8fa0a-f0fb-4d53-abce-d39e92df5201" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_e0924a88-bd5b-4e81-9d12-c6153abd028e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4dd72a58-7fa9-4141-b620-bfcbc8169d64" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_e0924a88-bd5b-4e81-9d12-c6153abd028e" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/StockbasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#StockbasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/StockbasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71994a31-307b-4659-8368-b40b3a739055" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71994a31-307b-4659-8368-b40b3a739055" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_74834b8c-9c26-44b4-a577-ce496b086b6f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71994a31-307b-4659-8368-b40b3a739055" xlink:to="loc_us-gaap_PlanNameAxis_74834b8c-9c26-44b4-a577-ce496b086b6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_74834b8c-9c26-44b4-a577-ce496b086b6f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_74834b8c-9c26-44b4-a577-ce496b086b6f" xlink:to="loc_us-gaap_PlanNameDomain_74834b8c-9c26-44b4-a577-ce496b086b6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_4915e136-55fe-4e47-955e-e0ca9ea8b09c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_74834b8c-9c26-44b4-a577-ce496b086b6f" xlink:to="loc_us-gaap_PlanNameDomain_4915e136-55fe-4e47-955e-e0ca9ea8b09c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OmnibusPlanMember_e1dbd50b-3074-4936-b2f6-e174ac82e1b8" xlink:href="fstr-20241231.xsd#fstr_OmnibusPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4915e136-55fe-4e47-955e-e0ca9ea8b09c" xlink:to="loc_fstr_OmnibusPlanMember_e1dbd50b-3074-4936-b2f6-e174ac82e1b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_EquityAndIncentiveCompensationPlanMember_14635118-d985-4c3c-8c5e-b32c0e8cfb67" xlink:href="fstr-20241231.xsd#fstr_EquityAndIncentiveCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4915e136-55fe-4e47-955e-e0ca9ea8b09c" xlink:to="loc_fstr_EquityAndIncentiveCompensationPlanMember_14635118-d985-4c3c-8c5e-b32c0e8cfb67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_EquityAndIncentivePlanMember_a52a0adf-ac29-4d50-9801-7091d06e5c7d" xlink:href="fstr-20241231.xsd#fstr_EquityAndIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4915e136-55fe-4e47-955e-e0ca9ea8b09c" xlink:to="loc_fstr_EquityAndIncentivePlanMember_a52a0adf-ac29-4d50-9801-7091d06e5c7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OmnibusIncentivePlanMember_2204bf5a-6ab6-4102-bc24-c34fed0f4c3b" xlink:href="fstr-20241231.xsd#fstr_OmnibusIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4915e136-55fe-4e47-955e-e0ca9ea8b09c" xlink:to="loc_fstr_OmnibusIncentivePlanMember_2204bf5a-6ab6-4102-bc24-c34fed0f4c3b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_OmnibusPlanAndEquityAndIncentivePlanMember_c6730b07-acf3-4fae-be33-093c22a269ac" xlink:href="fstr-20241231.xsd#fstr_OmnibusPlanAndEquityAndIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4915e136-55fe-4e47-955e-e0ca9ea8b09c" xlink:to="loc_fstr_OmnibusPlanAndEquityAndIncentivePlanMember_c6730b07-acf3-4fae-be33-093c22a269ac" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_67c2ab88-8ce9-4b12-aaeb-f1276951a7e1" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71994a31-307b-4659-8368-b40b3a739055" xlink:to="loc_srt_RangeAxis_67c2ab88-8ce9-4b12-aaeb-f1276951a7e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_67c2ab88-8ce9-4b12-aaeb-f1276951a7e1_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_67c2ab88-8ce9-4b12-aaeb-f1276951a7e1" xlink:to="loc_srt_RangeMember_67c2ab88-8ce9-4b12-aaeb-f1276951a7e1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c7d6d7dc-8c53-4ad1-80bb-4a535e83c562" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_67c2ab88-8ce9-4b12-aaeb-f1276951a7e1" xlink:to="loc_srt_RangeMember_c7d6d7dc-8c53-4ad1-80bb-4a535e83c562" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_c4f0beb4-8cc5-4174-9146-289966daa2ab" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_c7d6d7dc-8c53-4ad1-80bb-4a535e83c562" xlink:to="loc_srt_MaximumMember_c4f0beb4-8cc5-4174-9146-289966daa2ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_b7513342-5e4e-4f07-9a70-09c82cbf3647" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_c7d6d7dc-8c53-4ad1-80bb-4a535e83c562" xlink:to="loc_srt_MinimumMember_b7513342-5e4e-4f07-9a70-09c82cbf3647" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_a02c1a1e-2904-4705-9d32-2a102ed64eec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71994a31-307b-4659-8368-b40b3a739055" xlink:to="loc_us-gaap_AwardTypeAxis_a02c1a1e-2904-4705-9d32-2a102ed64eec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a02c1a1e-2904-4705-9d32-2a102ed64eec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_a02c1a1e-2904-4705-9d32-2a102ed64eec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a02c1a1e-2904-4705-9d32-2a102ed64eec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_a02c1a1e-2904-4705-9d32-2a102ed64eec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_c3ebca95-3160-483d-a961-4bac4369fdc3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:to="loc_us-gaap_EmployeeStockOptionMember_c3ebca95-3160-483d-a961-4bac4369fdc3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_FullyVestedAndRestrictedStockMember_7e9d6a81-cf78-48e3-b23b-e800e1cef816" xlink:href="fstr-20241231.xsd#fstr_FullyVestedAndRestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:to="loc_fstr_FullyVestedAndRestrictedStockMember_7e9d6a81-cf78-48e3-b23b-e800e1cef816" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_DeferredStockUnitsMember_9c0e8c8c-d744-451e-9132-7d78df448676" xlink:href="fstr-20241231.xsd#fstr_DeferredStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:to="loc_fstr_DeferredStockUnitsMember_9c0e8c8c-d744-451e-9132-7d78df448676" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_ed127f7f-b916-4c45-8d47-d76f7736331a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:to="loc_us-gaap_RestrictedStockMember_ed127f7f-b916-4c45-8d47-d76f7736331a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_3c569dbe-8ee9-4a15-b3c3-35d79deadbbc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:to="loc_us-gaap_PerformanceSharesMember_3c569dbe-8ee9-4a15-b3c3-35d79deadbbc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RistrictedStockAndPerformanceSharesCombinedMember_d3ddbd79-4ed8-4493-a74f-29e0a92e7813" xlink:href="fstr-20241231.xsd#fstr_RistrictedStockAndPerformanceSharesCombinedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0d57b102-52dc-4736-97bd-dfda634b1eab" xlink:to="loc_fstr_RistrictedStockAndPerformanceSharesCombinedMember_d3ddbd79-4ed8-4493-a74f-29e0a92e7813" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_c8ff7db0-80f9-4990-875f-d5538c4e07b4" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71994a31-307b-4659-8368-b40b3a739055" xlink:to="loc_srt_TitleOfIndividualAxis_c8ff7db0-80f9-4990-875f-d5538c4e07b4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_c8ff7db0-80f9-4990-875f-d5538c4e07b4_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_c8ff7db0-80f9-4990-875f-d5538c4e07b4" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_c8ff7db0-80f9-4990-875f-d5538c4e07b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_8458b223-a32e-41f5-ac38-ee81ab902d32" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_c8ff7db0-80f9-4990-875f-d5538c4e07b4" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_8458b223-a32e-41f5-ac38-ee81ab902d32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_DirectorMember_bf494dbc-8c27-4c88-8597-6a031afeafcd" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_DirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_8458b223-a32e-41f5-ac38-ee81ab902d32" xlink:to="loc_srt_DirectorMember_bf494dbc-8c27-4c88-8597-6a031afeafcd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_b77ac581-1395-46ee-aa6c-ca7abe625a79" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71994a31-307b-4659-8368-b40b3a739055" xlink:to="loc_us-gaap_VestingAxis_b77ac581-1395-46ee-aa6c-ca7abe625a79" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_b77ac581-1395-46ee-aa6c-ca7abe625a79_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_b77ac581-1395-46ee-aa6c-ca7abe625a79" xlink:to="loc_us-gaap_VestingDomain_b77ac581-1395-46ee-aa6c-ca7abe625a79_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_51dba429-c856-4d1a-b2a9-29bf903a3aff" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_b77ac581-1395-46ee-aa6c-ca7abe625a79" xlink:to="loc_us-gaap_VestingDomain_51dba429-c856-4d1a-b2a9-29bf903a3aff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_AchievementOfEBITDAMarginMember_63913e71-956e-40f8-9aca-73652bc05152" xlink:href="fstr-20241231.xsd#fstr_AchievementOfEBITDAMarginMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_51dba429-c856-4d1a-b2a9-29bf903a3aff" xlink:to="loc_fstr_AchievementOfEBITDAMarginMember_63913e71-956e-40f8-9aca-73652bc05152" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_AchievementOfStockPriceMilestoneMember_ca3af5cd-6d5f-4883-b6c1-aa86fc075eca" xlink:href="fstr-20241231.xsd#fstr_AchievementOfStockPriceMilestoneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_51dba429-c856-4d1a-b2a9-29bf903a3aff" xlink:to="loc_fstr_AchievementOfStockPriceMilestoneMember_ca3af5cd-6d5f-4883-b6c1-aa86fc075eca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_ed0cd06f-4393-4d93-9336-fe7473dd7c07" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_51dba429-c856-4d1a-b2a9-29bf903a3aff" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_ed0cd06f-4393-4d93-9336-fe7473dd7c07" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_ffd4d192-d8f2-4835-8af0-0ae0653bdbd2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_51dba429-c856-4d1a-b2a9-29bf903a3aff" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_ffd4d192-d8f2-4835-8af0-0ae0653bdbd2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_afe5384b-cd4d-4828-b75c-defadf86e882" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ShareBasedCompensation_afe5384b-cd4d-4828-b75c-defadf86e882" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_9eb7f174-15c2-4f7c-8937-c2a15e4a425a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_9eb7f174-15c2-4f7c-8937-c2a15e4a425a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_fa9c1a76-34c0-483b-9fa2-88f72a43109d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_fa9c1a76-34c0-483b-9fa2-88f72a43109d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_f45942ee-23f0-4217-b53c-076085107cc4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_f45942ee-23f0-4217-b53c-076085107cc4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_fe997423-7ba3-4e8b-bcf4-63d8de200233" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_fe997423-7ba3-4e8b-bcf4-63d8de200233" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_95d88ef2-f5be-40e4-830f-cf07ff2ec2cb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_95d88ef2-f5be-40e4-830f-cf07ff2ec2cb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_99bd0d1e-4497-4649-8c61-21f45f6b6192" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_99bd0d1e-4497-4649-8c61-21f45f6b6192" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_2a829830-071e-49f9-aa15-099593f50e5a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_2a829830-071e-49f9-aa15-099593f50e5a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_17cb001f-5d52-4739-aafe-6c369dd3787e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_17cb001f-5d52-4739-aafe-6c369dd3787e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_33590747-cd7d-400d-ad35-852f52021a5f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_33590747-cd7d-400d-ad35-852f52021a5f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_d1f0eb01-468e-4fec-8e57-0dc1cf325ea6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_d1f0eb01-468e-4fec-8e57-0dc1cf325ea6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetSharePrice_b0f3f9df-2239-4e35-9d97-08efa0a3ff14" xlink:href="fstr-20241231.xsd#fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetSharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_3b3ff08f-acbc-4938-802d-60c6dd879933" xlink:to="loc_fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetSharePrice_b0f3f9df-2239-4e35-9d97-08efa0a3ff14" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/StockbasedCompensationScheduleofRestrictedStockDeferredStockandPerformanceShareUnitsDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#StockbasedCompensationScheduleofRestrictedStockDeferredStockandPerformanceShareUnitsDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/StockbasedCompensationScheduleofRestrictedStockDeferredStockandPerformanceShareUnitsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c544fbe5-857a-4eb1-9c41-24444cba3c0e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c5663a9-3a34-4f4d-a44d-2818d34e8a5c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c544fbe5-857a-4eb1-9c41-24444cba3c0e" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c5663a9-3a34-4f4d-a44d-2818d34e8a5c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_8fa9df91-c438-4913-8e08-e744cbec8568" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c5663a9-3a34-4f4d-a44d-2818d34e8a5c" xlink:to="loc_us-gaap_AwardTypeAxis_8fa9df91-c438-4913-8e08-e744cbec8568" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8fa9df91-c438-4913-8e08-e744cbec8568_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_8fa9df91-c438-4913-8e08-e744cbec8568" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8fa9df91-c438-4913-8e08-e744cbec8568_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_39fe5e47-c010-46a3-84df-021b5ef8a7b4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_8fa9df91-c438-4913-8e08-e744cbec8568" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_39fe5e47-c010-46a3-84df-021b5ef8a7b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_c76ef244-ee06-4ae1-815c-7b23c9af85f0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_39fe5e47-c010-46a3-84df-021b5ef8a7b4" xlink:to="loc_us-gaap_RestrictedStockMember_c76ef244-ee06-4ae1-815c-7b23c9af85f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_DeferredStockUnitsMember_58215b3f-33b9-4990-8b44-6bf1d2ed7efa" xlink:href="fstr-20241231.xsd#fstr_DeferredStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_39fe5e47-c010-46a3-84df-021b5ef8a7b4" xlink:to="loc_fstr_DeferredStockUnitsMember_58215b3f-33b9-4990-8b44-6bf1d2ed7efa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_88b062b4-45e4-4bbc-89c5-46bb5a160657" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_39fe5e47-c010-46a3-84df-021b5ef8a7b4" xlink:to="loc_us-gaap_PerformanceSharesMember_88b062b4-45e4-4bbc-89c5-46bb5a160657" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_38cf1516-01ab-4d22-ac49-09813dcc1874" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c544fbe5-857a-4eb1-9c41-24444cba3c0e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_38cf1516-01ab-4d22-ac49-09813dcc1874" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_229fbcc8-0217-4585-a76d-4c1d395ce192" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_38cf1516-01ab-4d22-ac49-09813dcc1874" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_229fbcc8-0217-4585-a76d-4c1d395ce192" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_c42ed391-5f02-46bc-8796-488cd974dfc6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_38cf1516-01ab-4d22-ac49-09813dcc1874" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_c42ed391-5f02-46bc-8796-488cd974dfc6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_c3613e5b-daa2-4da3-8424-644aeb516ab3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_38cf1516-01ab-4d22-ac49-09813dcc1874" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_c3613e5b-daa2-4da3-8424-644aeb516ab3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdjustmentForIncentiveAwardsExpectedToVest_5996300d-d47b-4509-9e30-3083148a75d0" xlink:href="fstr-20241231.xsd#fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdjustmentForIncentiveAwardsExpectedToVest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_38cf1516-01ab-4d22-ac49-09813dcc1874" xlink:to="loc_fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdjustmentForIncentiveAwardsExpectedToVest_5996300d-d47b-4509-9e30-3083148a75d0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_46507ba0-e8af-48e4-b891-b2e62b322937" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_38cf1516-01ab-4d22-ac49-09813dcc1874" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_46507ba0-e8af-48e4-b891-b2e62b322937" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_b6fa6c7b-9770-4bbb-96ee-f5583ceeb582" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_9d8aae57-d6f4-4754-a360-3db161b6d0e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c544fbe5-857a-4eb1-9c41-24444cba3c0e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_9d8aae57-d6f4-4754-a360-3db161b6d0e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_9e9eff1c-cda3-49de-a914-950d12702277" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_9d8aae57-d6f4-4754-a360-3db161b6d0e6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_9e9eff1c-cda3-49de-a914-950d12702277" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_9d161b93-2701-4a2a-92d0-0113edee2d7f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_9d8aae57-d6f4-4754-a360-3db161b6d0e6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_9d161b93-2701-4a2a-92d0-0113edee2d7f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_935e4f87-d7a1-491d-8073-deb2d520380e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_9d8aae57-d6f4-4754-a360-3db161b6d0e6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_935e4f87-d7a1-491d-8073-deb2d520380e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdjustmentForIncentiveAwardsExpectedToVestWeightedAverageGrantDateFairValue_2a61d6c6-bd0d-4830-84e8-e209c4f5f64e" xlink:href="fstr-20241231.xsd#fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdjustmentForIncentiveAwardsExpectedToVestWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_9d8aae57-d6f4-4754-a360-3db161b6d0e6" xlink:to="loc_fstr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdjustmentForIncentiveAwardsExpectedToVestWeightedAverageGrantDateFairValue_2a61d6c6-bd0d-4830-84e8-e209c4f5f64e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_f44b53d5-696d-4ce5-8e48-3305ac80ab6e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_9d8aae57-d6f4-4754-a360-3db161b6d0e6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_f44b53d5-696d-4ce5-8e48-3305ac80ab6e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_0359fc72-2746-48e7-b892-06a73c16cc79" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#FairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_738e16ba-de16-4869-b933-7c8b5b329b5c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_5c3503f0-e722-4fe1-a153-d58cfa796827" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_738e16ba-de16-4869-b933-7c8b5b329b5c" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_5c3503f0-e722-4fe1-a153-d58cfa796827" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_4c38c543-bfbc-42da-a79f-c91bffba451f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_5c3503f0-e722-4fe1-a153-d58cfa796827" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_4c38c543-bfbc-42da-a79f-c91bffba451f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_4c38c543-bfbc-42da-a79f-c91bffba451f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_4c38c543-bfbc-42da-a79f-c91bffba451f" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_4c38c543-bfbc-42da-a79f-c91bffba451f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_95524959-9dd2-4b48-b8fb-c838bd45934d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_4c38c543-bfbc-42da-a79f-c91bffba451f" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_95524959-9dd2-4b48-b8fb-c838bd45934d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SwapMember_b2e073e1-87e5-4e9e-a721-8bddf9a91d52" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_95524959-9dd2-4b48-b8fb-c838bd45934d" xlink:to="loc_us-gaap_SwapMember_b2e073e1-87e5-4e9e-a721-8bddf9a91d52" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_Swap2Member_b92a8e4b-2564-4927-ad9b-db515b8c3910" xlink:href="fstr-20241231.xsd#fstr_Swap2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_95524959-9dd2-4b48-b8fb-c838bd45934d" xlink:to="loc_fstr_Swap2Member_b92a8e4b-2564-4927-ad9b-db515b8c3910" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_94a7a223-916c-4d5e-b621-6b90d3d2a700" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_738e16ba-de16-4869-b933-7c8b5b329b5c" xlink:to="loc_us-gaap_DerivativeNotionalAmount_94a7a223-916c-4d5e-b621-6b90d3d2a700" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_05dcd010-45d3-4c13-9fef-ec8d28728821" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_738e16ba-de16-4869-b933-7c8b5b329b5c" xlink:to="loc_us-gaap_InvestmentIncomeInterest_05dcd010-45d3-4c13-9fef-ec8d28728821" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/FairValueMeasurementsScheduleofFairValueAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#FairValueMeasurementsScheduleofFairValueAssetsandLiabilitiesMeasuredonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/FairValueMeasurementsScheduleofFairValueAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_12453b7b-b61c-40f2-be8e-9817a346473c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5356b1b-6b01-425d-9418-22792fd6c964" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_12453b7b-b61c-40f2-be8e-9817a346473c" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5356b1b-6b01-425d-9418-22792fd6c964" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_18004120-9a39-475c-a704-6722df6cb81a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5356b1b-6b01-425d-9418-22792fd6c964" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_18004120-9a39-475c-a704-6722df6cb81a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_18004120-9a39-475c-a704-6722df6cb81a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_18004120-9a39-475c-a704-6722df6cb81a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_18004120-9a39-475c-a704-6722df6cb81a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_81028d32-3e48-4ab9-b3a2-67f99e7b3b0b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_18004120-9a39-475c-a704-6722df6cb81a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_81028d32-3e48-4ab9-b3a2-67f99e7b3b0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_d674e7eb-abe6-4c74-a31b-f87cf0fbed6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_81028d32-3e48-4ab9-b3a2-67f99e7b3b0b" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_d674e7eb-abe6-4c74-a31b-f87cf0fbed6c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_48cf1fc4-c3a8-493f-ad0a-5d9ee26256b3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_81028d32-3e48-4ab9-b3a2-67f99e7b3b0b" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_48cf1fc4-c3a8-493f-ad0a-5d9ee26256b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_78b89de5-3318-463f-b1ea-4d5092ccb930" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_81028d32-3e48-4ab9-b3a2-67f99e7b3b0b" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_78b89de5-3318-463f-b1ea-4d5092ccb930" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetsCurrent_bcb2b2e9-c6fc-4e0c-9383-6ce79e18862b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_12453b7b-b61c-40f2-be8e-9817a346473c" xlink:to="loc_us-gaap_DerivativeAssetsCurrent_bcb2b2e9-c6fc-4e0c-9383-6ce79e18862b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetCurrentStatementOfFinancialPositionExtensibleEnumeration_4e0545ac-5468-4825-993e-786953a564d3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeAssetCurrentStatementOfFinancialPositionExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_12453b7b-b61c-40f2-be8e-9817a346473c" xlink:to="loc_us-gaap_DerivativeAssetCurrentStatementOfFinancialPositionExtensibleEnumeration_4e0545ac-5468-4825-993e-786953a564d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_4a99ab48-8b12-4cc1-b3d4-f1c2749df980" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_12453b7b-b61c-40f2-be8e-9817a346473c" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_4a99ab48-8b12-4cc1-b3d4-f1c2749df980" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15971d07-65bc-4fba-8d3f-230d358ac062" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_27f930a2-8ae9-49e2-957d-077e5dfbab35" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15971d07-65bc-4fba-8d3f-230d358ac062" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_27f930a2-8ae9-49e2-957d-077e5dfbab35" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_270fb685-d032-45c6-a1ec-94157ae5bef3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_27f930a2-8ae9-49e2-957d-077e5dfbab35" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_270fb685-d032-45c6-a1ec-94157ae5bef3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_270fb685-d032-45c6-a1ec-94157ae5bef3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_270fb685-d032-45c6-a1ec-94157ae5bef3" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_270fb685-d032-45c6-a1ec-94157ae5bef3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_3a8abd46-e5b2-48cc-a477-3673b3d25487" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_270fb685-d032-45c6-a1ec-94157ae5bef3" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_3a8abd46-e5b2-48cc-a477-3673b3d25487" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_eb3b786e-82cf-4559-a640-85191422454f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_3a8abd46-e5b2-48cc-a477-3673b3d25487" xlink:to="loc_us-gaap_DomesticPlanMember_eb3b786e-82cf-4559-a640-85191422454f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_e7a04c78-7836-4d0b-a4de-b2ef0a6cb0b1" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_eb3b786e-82cf-4559-a640-85191422454f" xlink:to="loc_country_US_e7a04c78-7836-4d0b-a4de-b2ef0a6cb0b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_e151a264-ef77-4693-969b-db32fa263804" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_3a8abd46-e5b2-48cc-a477-3673b3d25487" xlink:to="loc_us-gaap_ForeignPlanMember_e151a264-ef77-4693-969b-db32fa263804" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_07ddb81b-6b19-44f3-8f79-b255ce9c1d3d" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_e151a264-ef77-4693-969b-db32fa263804" xlink:to="loc_country_CA_07ddb81b-6b19-44f3-8f79-b255ce9c1d3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_6414510e-e092-407b-a824-d9d77316278c" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_e151a264-ef77-4693-969b-db32fa263804" xlink:to="loc_country_GB_6414510e-e092-407b-a824-d9d77316278c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_NumberOfDefinedContributionPlans_1d95b71e-54f5-4af4-9bcc-501760919bea" xlink:href="fstr-20241231.xsd#fstr_NumberOfDefinedContributionPlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15971d07-65bc-4fba-8d3f-230d358ac062" xlink:to="loc_fstr_NumberOfDefinedContributionPlans_1d95b71e-54f5-4af4-9bcc-501760919bea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_e614f9c5-2a0c-4979-88a5-097d2da6e078" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanContributionsByEmployer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15971d07-65bc-4fba-8d3f-230d358ac062" xlink:to="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_e614f9c5-2a0c-4979-88a5-097d2da6e078" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanSettlementsBenefitObligation_27066180-23f7-450a-95b0-518739cbf4ea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanSettlementsBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15971d07-65bc-4fba-8d3f-230d358ac062" xlink:to="loc_us-gaap_DefinedBenefitPlanSettlementsBenefitObligation_27066180-23f7-450a-95b0-518739cbf4ea" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths_e6a38e7c-7c5c-4ace-b9e7-c0f85f65e864" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_15971d07-65bc-4fba-8d3f-230d358ac062" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths_e6a38e7c-7c5c-4ace-b9e7-c0f85f65e864" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansScheduleofBenefitObligationFairValueofAssetsandFundedStatusofthePlansDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansScheduleofBenefitObligationFairValueofAssetsandFundedStatusofthePlansDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansScheduleofBenefitObligationFairValueofAssetsandFundedStatusofthePlansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a3ebf6e9-da89-4e73-bce8-dad7cbc7ab5e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_a7cd165d-a471-4cfd-8b17-637abd8c69ad" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a3ebf6e9-da89-4e73-bce8-dad7cbc7ab5e" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_a7cd165d-a471-4cfd-8b17-637abd8c69ad" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_a807554b-87cc-48b0-8d8e-ef3513883a52" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_a7cd165d-a471-4cfd-8b17-637abd8c69ad" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_a807554b-87cc-48b0-8d8e-ef3513883a52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_a807554b-87cc-48b0-8d8e-ef3513883a52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_a807554b-87cc-48b0-8d8e-ef3513883a52" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_a807554b-87cc-48b0-8d8e-ef3513883a52_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_4bad34d2-60f3-45e7-848b-c160e7cb9b3d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_a807554b-87cc-48b0-8d8e-ef3513883a52" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_4bad34d2-60f3-45e7-848b-c160e7cb9b3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_8fa2119f-2e75-4670-b5c5-6b6aa3622784" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_4bad34d2-60f3-45e7-848b-c160e7cb9b3d" xlink:to="loc_us-gaap_DomesticPlanMember_8fa2119f-2e75-4670-b5c5-6b6aa3622784" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_5a99e8d8-a54a-4a2f-b725-c6829b9cf8d4" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_8fa2119f-2e75-4670-b5c5-6b6aa3622784" xlink:to="loc_country_US_5a99e8d8-a54a-4a2f-b725-c6829b9cf8d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_2f708dcc-ef11-4f7c-bd4c-da4a42033e25" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_4bad34d2-60f3-45e7-848b-c160e7cb9b3d" xlink:to="loc_us-gaap_ForeignPlanMember_2f708dcc-ef11-4f7c-bd4c-da4a42033e25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_2ca972e0-4d47-49bf-93f7-8d6c6802fab5" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_2f708dcc-ef11-4f7c-bd4c-da4a42033e25" xlink:to="loc_country_GB_2ca972e0-4d47-49bf-93f7-8d6c6802fab5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a3ebf6e9-da89-4e73-bce8-dad7cbc7ab5e" xlink:to="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_827397f1-fad4-4700-8370-574843cf064e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligation_827397f1-fad4-4700-8370-574843cf064e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_b50bd981-2c80-4bf9-af66-c6d6cc303d72" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanServiceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_b50bd981-2c80-4bf9-af66-c6d6cc303d72" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_168b086a-086e-4ca9-83c2-753f43a00ff4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_168b086a-086e-4ca9-83c2-753f43a00ff4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActuarialGainLoss_4245d5e5-5ab0-47b8-b901-439b8a85e6c3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanActuarialGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:to="loc_us-gaap_DefinedBenefitPlanActuarialGainLoss_4245d5e5-5ab0-47b8-b901-439b8a85e6c3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_7b13a022-f0ce-42aa-9f4b-140764538134" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_7b13a022-f0ce-42aa-9f4b-140764538134" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation_241084c5-7ff8-4c71-8490-e0315a0d9004" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:to="loc_us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation_241084c5-7ff8-4c71-8490-e0315a0d9004" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligationPaymentForSettlement_05435515-cb7d-4b0c-9388-7eda2cf6d012" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanBenefitObligationPaymentForSettlement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_f9f51899-4bf1-491b-b02e-c736f65aa41d" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligationPaymentForSettlement_05435515-cb7d-4b0c-9388-7eda2cf6d012" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_7e166bc1-d7a4-48d9-b5b5-e130e67a9842" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a3ebf6e9-da89-4e73-bce8-dad7cbc7ab5e" xlink:to="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_5d1e3a1e-9bdb-4ed3-9b36-d8321b810375" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_5d1e3a1e-9bdb-4ed3-9b36-d8321b810375" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets_a18f1703-e730-44c1-990d-54af243cd1b8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:to="loc_us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets_a18f1703-e730-44c1-990d-54af243cd1b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_064a1270-4e4a-4079-b788-815dbeede855" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanContributionsByEmployer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:to="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_064a1270-4e4a-4079-b788-815dbeede855" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid_a1ff08ad-8fde-43b8-b4ae-7179f3315cb4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid_a1ff08ad-8fde-43b8-b4ae-7179f3315cb4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_DefinedBenefitPlanPlanAssetsReversion_c99fb54e-4280-4a63-b6fc-6ef8bbe9287a" xlink:href="fstr-20241231.xsd#fstr_DefinedBenefitPlanPlanAssetsReversion"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:to="loc_fstr_DefinedBenefitPlanPlanAssetsReversion_c99fb54e-4280-4a63-b6fc-6ef8bbe9287a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsForeignCurrencyTranslationGainLoss_afdd2b7d-4971-4b40-a8b7-e3b75d6a23c4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanPlanAssetsForeignCurrencyTranslationGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsForeignCurrencyTranslationGainLoss_afdd2b7d-4971-4b40-a8b7-e3b75d6a23c4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanSettlementsPlanAssets_e992d7f2-adb8-4542-a3be-f380be54a5d0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanSettlementsPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_83d1754a-f2fc-446e-af58-f826af1eb734" xlink:to="loc_us-gaap_DefinedBenefitPlanSettlementsPlanAssets_e992d7f2-adb8-4542-a3be-f380be54a5d0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_5ebcab81-b324-42c8-a379-3200acd50b7e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_88f46823-8611-40fc-b80d-d1b9efdfffe5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanFundedStatusOfPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a3ebf6e9-da89-4e73-bce8-dad7cbc7ab5e" xlink:to="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_88f46823-8611-40fc-b80d-d1b9efdfffe5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_1e90e2e2-47b0-4e9f-8632-440771f344d0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a3ebf6e9-da89-4e73-bce8-dad7cbc7ab5e" xlink:to="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_1e90e2e2-47b0-4e9f-8632-440771f344d0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent_f42a4944-67ea-4b62-9c62-f3c411ecf7ab" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_1e90e2e2-47b0-4e9f-8632-440771f344d0" xlink:to="loc_us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent_f42a4944-67ea-4b62-9c62-f3c411ecf7ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssetsForPlanBenefitsNoncurrent_347c44b3-db4c-43da-8b97-dd6fa23fd3f5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAssetsForPlanBenefitsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_1e90e2e2-47b0-4e9f-8632-440771f344d0" xlink:to="loc_us-gaap_DefinedBenefitPlanAssetsForPlanBenefitsNoncurrent_347c44b3-db4c-43da-8b97-dd6fa23fd3f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTaxAbstract_9caa26c2-439d-4c5c-97d4-af21c7eb3298" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a3ebf6e9-da89-4e73-bce8-dad7cbc7ab5e" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTaxAbstract_9caa26c2-439d-4c5c-97d4-af21c7eb3298" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax_f55fbbbb-8557-4fa8-b73f-91fdb7848536" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTaxAbstract_9caa26c2-439d-4c5c-97d4-af21c7eb3298" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax_f55fbbbb-8557-4fa8-b73f-91fdb7848536" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax_205c3626-ee92-4653-89b0-368f1a087e39" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTaxAbstract_9caa26c2-439d-4c5c-97d4-af21c7eb3298" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax_205c3626-ee92-4653-89b0-368f1a087e39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax_b22ec2c1-6756-4e1f-8f79-22da52a81f47" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTaxAbstract_9caa26c2-439d-4c5c-97d4-af21c7eb3298" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax_b22ec2c1-6756-4e1f-8f79-22da52a81f47" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansScheduleofNetBenefitCostsDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansScheduleofNetBenefitCostsDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansScheduleofNetBenefitCostsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_36c2bf82-b6f7-408d-bf64-fc73c23b1679" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_43131e1f-0fef-46f5-9f19-a865721a1878" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_36c2bf82-b6f7-408d-bf64-fc73c23b1679" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_43131e1f-0fef-46f5-9f19-a865721a1878" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_90199dd1-ef0f-46b1-96ae-1f30a4fea025" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_43131e1f-0fef-46f5-9f19-a865721a1878" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_90199dd1-ef0f-46b1-96ae-1f30a4fea025" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_90199dd1-ef0f-46b1-96ae-1f30a4fea025_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_90199dd1-ef0f-46b1-96ae-1f30a4fea025" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_90199dd1-ef0f-46b1-96ae-1f30a4fea025_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_356d9ec8-724b-4ba6-821a-f80bf7556ccc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_90199dd1-ef0f-46b1-96ae-1f30a4fea025" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_356d9ec8-724b-4ba6-821a-f80bf7556ccc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_a3441f79-4234-4082-8404-6dc5bc7511a5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_356d9ec8-724b-4ba6-821a-f80bf7556ccc" xlink:to="loc_us-gaap_DomesticPlanMember_a3441f79-4234-4082-8404-6dc5bc7511a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_1e1f75aa-84db-464e-bd50-6bffc7f09465" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_a3441f79-4234-4082-8404-6dc5bc7511a5" xlink:to="loc_country_US_1e1f75aa-84db-464e-bd50-6bffc7f09465" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_35410d7e-294a-49a2-91ba-5019aa876a7f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_356d9ec8-724b-4ba6-821a-f80bf7556ccc" xlink:to="loc_us-gaap_ForeignPlanMember_35410d7e-294a-49a2-91ba-5019aa876a7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_41ec5318-d826-45d0-8999-b2d942e65ccd" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_35410d7e-294a-49a2-91ba-5019aa876a7f" xlink:to="loc_country_GB_41ec5318-d826-45d0-8999-b2d942e65ccd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_36c2bf82-b6f7-408d-bf64-fc73c23b1679" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_763ccbbb-1553-4913-97c8-86f628c03289" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_763ccbbb-1553-4913-97c8-86f628c03289" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_27120610-46df-4d48-81c3-c73b129b3e92" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_27120610-46df-4d48-81c3-c73b129b3e92" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_4cf5e80b-315d-4bd7-863f-aacb808d07de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_4cf5e80b-315d-4bd7-863f-aacb808d07de" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_ad1c7e7d-a04d-419e-ac77-d0e30e2e7624" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_ad1c7e7d-a04d-419e-ac77-d0e30e2e7624" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_DefinedBenefitPlanNetPeriodicBenefitCostCreditExcludingGainLossFromSettlements_b3ef08e4-c67c-45e6-9e8e-b25891b8f386" xlink:href="fstr-20241231.xsd#fstr_DefinedBenefitPlanNetPeriodicBenefitCostCreditExcludingGainLossFromSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:to="loc_fstr_DefinedBenefitPlanNetPeriodicBenefitCostCreditExcludingGainLossFromSettlements_b3ef08e4-c67c-45e6-9e8e-b25891b8f386" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_335198b8-d4e0-468f-8095-b5cd13e30c9f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:to="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_335198b8-d4e0-468f-8095-b5cd13e30c9f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_40116a2e-b4b3-4b8f-b7d1-cbddd496f0a7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_e2ff06ab-95f5-400b-a56d-bbb0daa2d520" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_40116a2e-b4b3-4b8f-b7d1-cbddd496f0a7" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansScheduleofActuarialAssumptionsDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansScheduleofActuarialAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansScheduleofActuarialAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_51b137a3-a53c-4bcb-ac58-a6c3577f537d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_5f197cdf-416d-4ab5-83eb-eaa9d4780c96" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_51b137a3-a53c-4bcb-ac58-a6c3577f537d" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_5f197cdf-416d-4ab5-83eb-eaa9d4780c96" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_16c291c8-aa31-4c61-a27a-716b7d209007" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_5f197cdf-416d-4ab5-83eb-eaa9d4780c96" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_16c291c8-aa31-4c61-a27a-716b7d209007" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_16c291c8-aa31-4c61-a27a-716b7d209007_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_16c291c8-aa31-4c61-a27a-716b7d209007" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_16c291c8-aa31-4c61-a27a-716b7d209007_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_6c23d5ab-6b64-47ce-b2fd-23b9a8ce814a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_16c291c8-aa31-4c61-a27a-716b7d209007" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_6c23d5ab-6b64-47ce-b2fd-23b9a8ce814a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_29173774-a1d5-478d-a16c-4a9fd92fc40e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_6c23d5ab-6b64-47ce-b2fd-23b9a8ce814a" xlink:to="loc_us-gaap_DomesticPlanMember_29173774-a1d5-478d-a16c-4a9fd92fc40e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_20809643-6434-470b-979e-11842df1995f" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_29173774-a1d5-478d-a16c-4a9fd92fc40e" xlink:to="loc_country_US_20809643-6434-470b-979e-11842df1995f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_5cab9f13-0e2e-4c0b-8cb6-a0b4165b684e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_6c23d5ab-6b64-47ce-b2fd-23b9a8ce814a" xlink:to="loc_us-gaap_ForeignPlanMember_5cab9f13-0e2e-4c0b-8cb6-a0b4165b684e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_f69d8a68-05e1-45f3-bfd8-998622d092af" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_5cab9f13-0e2e-4c0b-8cb6-a0b4165b684e" xlink:to="loc_country_GB_f69d8a68-05e1-45f3-bfd8-998622d092af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_6a7b4bde-3f30-4d41-9bf0-34285ddbf5fa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_51b137a3-a53c-4bcb-ac58-a6c3577f537d" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_6a7b4bde-3f30-4d41-9bf0-34285ddbf5fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_a99716c9-a512-4cb0-9195-2932e08310f5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_51b137a3-a53c-4bcb-ac58-a6c3577f537d" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_a99716c9-a512-4cb0-9195-2932e08310f5" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansScheduleofAccumulatedBenefitObligationsinExcessofPlanAssetDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansScheduleofAccumulatedBenefitObligationsinExcessofPlanAssetDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansScheduleofAccumulatedBenefitObligationsinExcessofPlanAssetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_9520c0ef-f85a-4c18-89ec-f641fb156f2a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_c0ee6f69-b365-4a74-ae58-8c2d49c05130" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_9520c0ef-f85a-4c18-89ec-f641fb156f2a" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_c0ee6f69-b365-4a74-ae58-8c2d49c05130" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_a2d01f7d-5a8e-4fc8-ab8d-63322cc6af55" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_c0ee6f69-b365-4a74-ae58-8c2d49c05130" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_a2d01f7d-5a8e-4fc8-ab8d-63322cc6af55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_a2d01f7d-5a8e-4fc8-ab8d-63322cc6af55_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_a2d01f7d-5a8e-4fc8-ab8d-63322cc6af55" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_a2d01f7d-5a8e-4fc8-ab8d-63322cc6af55_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_49834a0f-dacf-40f4-b349-89b97e30194d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_a2d01f7d-5a8e-4fc8-ab8d-63322cc6af55" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_49834a0f-dacf-40f4-b349-89b97e30194d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_d58a06e4-89f5-4f95-8425-3aabdc7ad39c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_49834a0f-dacf-40f4-b349-89b97e30194d" xlink:to="loc_us-gaap_DomesticPlanMember_d58a06e4-89f5-4f95-8425-3aabdc7ad39c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_115b2a60-144e-4676-95b6-874065e9b0de" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_d58a06e4-89f5-4f95-8425-3aabdc7ad39c" xlink:to="loc_country_US_115b2a60-144e-4676-95b6-874065e9b0de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_5ce8c5b1-4ac8-4172-bf85-bb31adf0d2ef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_49834a0f-dacf-40f4-b349-89b97e30194d" xlink:to="loc_us-gaap_ForeignPlanMember_5ce8c5b1-4ac8-4172-bf85-bb31adf0d2ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_089d82f9-2534-446d-8c92-237412bc14b5" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_5ce8c5b1-4ac8-4172-bf85-bb31adf0d2ef" xlink:to="loc_country_GB_089d82f9-2534-446d-8c92-237412bc14b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_fc482a13-d841-410a-b2e7-7b046b102a3d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_9520c0ef-f85a-4c18-89ec-f641fb156f2a" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligation_fc482a13-d841-410a-b2e7-7b046b102a3d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation_28672d9f-dcf6-4ff9-a70c-99f0acadade2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_9520c0ef-f85a-4c18-89ec-f641fb156f2a" xlink:to="loc_us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation_28672d9f-dcf6-4ff9-a70c-99f0acadade2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_200f7489-b9b9-444e-ba4d-f7fdaade1616" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_9520c0ef-f85a-4c18-89ec-f641fb156f2a" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_200f7489-b9b9-444e-ba4d-f7fdaade1616" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansScheduleofPlanAssetInvestmentAllocationPercentageDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansScheduleofPlanAssetInvestmentAllocationPercentageDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansScheduleofPlanAssetInvestmentAllocationPercentageDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_34de233e-2e4a-43e6-9674-deb856fb669f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_cfd7bb84-2cdf-4cc7-8e6a-25a4f74dfeef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_34de233e-2e4a-43e6-9674-deb856fb669f" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_cfd7bb84-2cdf-4cc7-8e6a-25a4f74dfeef" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_ca11271b-4611-4a28-b564-55c63f04b92c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_cfd7bb84-2cdf-4cc7-8e6a-25a4f74dfeef" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_ca11271b-4611-4a28-b564-55c63f04b92c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_ca11271b-4611-4a28-b564-55c63f04b92c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_ca11271b-4611-4a28-b564-55c63f04b92c" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_ca11271b-4611-4a28-b564-55c63f04b92c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_9bd2945e-7c0c-46a4-b9ab-0d645d53c27a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_ca11271b-4611-4a28-b564-55c63f04b92c" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_9bd2945e-7c0c-46a4-b9ab-0d645d53c27a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_83b79f58-265b-492b-9577-bd2f411ec960" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_9bd2945e-7c0c-46a4-b9ab-0d645d53c27a" xlink:to="loc_us-gaap_DomesticPlanMember_83b79f58-265b-492b-9577-bd2f411ec960" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_b707e59c-f87e-43c3-907e-f152d39653cf" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_83b79f58-265b-492b-9577-bd2f411ec960" xlink:to="loc_country_US_b707e59c-f87e-43c3-907e-f152d39653cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_910cceb1-468c-412e-a3b1-c6be2eb82f14" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_9bd2945e-7c0c-46a4-b9ab-0d645d53c27a" xlink:to="loc_us-gaap_ForeignPlanMember_910cceb1-468c-412e-a3b1-c6be2eb82f14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_e20c8b42-c278-452a-87fe-6d0b422e2d63" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_910cceb1-468c-412e-a3b1-c6be2eb82f14" xlink:to="loc_country_GB_e20c8b42-c278-452a-87fe-6d0b422e2d63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_b629bde9-c180-496e-b3ba-b18ac8a929c9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_cfd7bb84-2cdf-4cc7-8e6a-25a4f74dfeef" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_b629bde9-c180-496e-b3ba-b18ac8a929c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_b629bde9-c180-496e-b3ba-b18ac8a929c9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_b629bde9-c180-496e-b3ba-b18ac8a929c9" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_b629bde9-c180-496e-b3ba-b18ac8a929c9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_b629bde9-c180-496e-b3ba-b18ac8a929c9" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsMember_7ccc6b77-386e-4aa8-b5b1-069548f5b752" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:to="loc_us-gaap_CashAndCashEquivalentsMember_7ccc6b77-386e-4aa8-b5b1-069548f5b752" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedIncomeFundsMember_96f32b07-fb84-49d2-9370-8d02a13d124c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FixedIncomeFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:to="loc_us-gaap_FixedIncomeFundsMember_96f32b07-fb84-49d2-9370-8d02a13d124c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityFundsMember_7d06efe5-3275-4bbb-9d8d-0a68dc942b05" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:to="loc_us-gaap_EquityFundsMember_7d06efe5-3275-4bbb-9d8d-0a68dc942b05" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_8908b9b0-59a4-4242-bfcd-05cd4088918f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:to="loc_us-gaap_EquitySecuritiesMember_8908b9b0-59a4-4242-bfcd-05cd4088918f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_1db0c564-e1d5-4a70-a968-9a86c76d9277" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:to="loc_us-gaap_CommercialRealEstateMember_1db0c564-e1d5-4a70-a968-9a86c76d9277" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_976d359c-3ec5-468b-b13b-082c8f25675f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_b7e4c0df-edad-4ee0-ab0c-d5b64965d444" xlink:to="loc_us-gaap_MunicipalBondsMember_976d359c-3ec5-468b-b13b-082c8f25675f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_a82be982-6221-4965-b829-aee316c88a70" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_cfd7bb84-2cdf-4cc7-8e6a-25a4f74dfeef" xlink:to="loc_srt_RangeAxis_a82be982-6221-4965-b829-aee316c88a70" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a82be982-6221-4965-b829-aee316c88a70_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_a82be982-6221-4965-b829-aee316c88a70" xlink:to="loc_srt_RangeMember_a82be982-6221-4965-b829-aee316c88a70_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_cbf7e69b-eac5-483a-add5-48e2ac669b19" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_a82be982-6221-4965-b829-aee316c88a70" xlink:to="loc_srt_RangeMember_cbf7e69b-eac5-483a-add5-48e2ac669b19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_4de53856-98d0-4584-bd73-0513f35d5a33" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_cbf7e69b-eac5-483a-add5-48e2ac669b19" xlink:to="loc_srt_MinimumMember_4de53856-98d0-4584-bd73-0513f35d5a33" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_c948920b-694c-437d-8f08-644f0bf0bccc" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_cbf7e69b-eac5-483a-add5-48e2ac669b19" xlink:to="loc_srt_MaximumMember_c948920b-694c-437d-8f08-644f0bf0bccc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInformationAboutPlanAssetsAbstract_f9abfd92-5bef-49f4-ad2e-def659aab809" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanInformationAboutPlanAssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_34de233e-2e4a-43e6-9674-deb856fb669f" xlink:to="loc_us-gaap_DefinedBenefitPlanInformationAboutPlanAssetsAbstract_f9abfd92-5bef-49f4-ad2e-def659aab809" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_1dafa4b6-738f-4f13-af97-80c7b443ac31" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanInformationAboutPlanAssetsAbstract_f9abfd92-5bef-49f4-ad2e-def659aab809" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_1dafa4b6-738f-4f13-af97-80c7b443ac31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations_bf29383b-6feb-4adb-8e43-178f5baf63ea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanInformationAboutPlanAssetsAbstract_f9abfd92-5bef-49f4-ad2e-def659aab809" xlink:to="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations_bf29383b-6feb-4adb-8e43-178f5baf63ea" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansScheduleofInformationAboutPlanAssetsDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansScheduleofInformationAboutPlanAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansScheduleofInformationAboutPlanAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_51bb30fe-58cc-4015-bce5-c2e6c6c8cdf6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_0cf9524d-6019-49c5-9ce1-aad05491fb04" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_51bb30fe-58cc-4015-bce5-c2e6c6c8cdf6" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_0cf9524d-6019-49c5-9ce1-aad05491fb04" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_7d4bc857-c967-4258-8743-87efac89431e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_0cf9524d-6019-49c5-9ce1-aad05491fb04" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_7d4bc857-c967-4258-8743-87efac89431e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_7d4bc857-c967-4258-8743-87efac89431e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_7d4bc857-c967-4258-8743-87efac89431e" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_7d4bc857-c967-4258-8743-87efac89431e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_7f0611f0-d7a1-4d0d-9ad1-f123696edbef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_7d4bc857-c967-4258-8743-87efac89431e" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_7f0611f0-d7a1-4d0d-9ad1-f123696edbef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_a922c892-82fe-46c8-99fe-724f65e9b721" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_7f0611f0-d7a1-4d0d-9ad1-f123696edbef" xlink:to="loc_us-gaap_DomesticPlanMember_a922c892-82fe-46c8-99fe-724f65e9b721" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_4ae5e773-4f0e-4d06-9b33-e22acf1c8e5d" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_a922c892-82fe-46c8-99fe-724f65e9b721" xlink:to="loc_country_US_4ae5e773-4f0e-4d06-9b33-e22acf1c8e5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_bbe335b3-941c-42b7-b424-db3ba6404e00" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_7f0611f0-d7a1-4d0d-9ad1-f123696edbef" xlink:to="loc_us-gaap_ForeignPlanMember_bbe335b3-941c-42b7-b424-db3ba6404e00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_66b41eb6-f164-4b2e-9c1b-a979f5472cb2" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_bbe335b3-941c-42b7-b424-db3ba6404e00" xlink:to="loc_country_GB_66b41eb6-f164-4b2e-9c1b-a979f5472cb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_a327f60f-e4aa-44ef-b5c3-9fdeaace688f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_0cf9524d-6019-49c5-9ce1-aad05491fb04" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_a327f60f-e4aa-44ef-b5c3-9fdeaace688f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_a327f60f-e4aa-44ef-b5c3-9fdeaace688f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_a327f60f-e4aa-44ef-b5c3-9fdeaace688f" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_a327f60f-e4aa-44ef-b5c3-9fdeaace688f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_c3f5d9a4-3454-4310-a366-6818f4a0ba5c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_a327f60f-e4aa-44ef-b5c3-9fdeaace688f" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_c3f5d9a4-3454-4310-a366-6818f4a0ba5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsMember_ba43bdbd-8ab8-4040-b833-8a84032c17b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_c3f5d9a4-3454-4310-a366-6818f4a0ba5c" xlink:to="loc_us-gaap_CashAndCashEquivalentsMember_ba43bdbd-8ab8-4040-b833-8a84032c17b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedIncomeFundsMember_bc9713f2-17d1-43fd-a046-7217939dbb87" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FixedIncomeFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_c3f5d9a4-3454-4310-a366-6818f4a0ba5c" xlink:to="loc_us-gaap_FixedIncomeFundsMember_bc9713f2-17d1-43fd-a046-7217939dbb87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateBondSecuritiesMember_38292616-64ff-4971-abb5-ee7d6f8658ba" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CorporateBondSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedIncomeFundsMember_bc9713f2-17d1-43fd-a046-7217939dbb87" xlink:to="loc_us-gaap_CorporateBondSecuritiesMember_38292616-64ff-4971-abb5-ee7d6f8658ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_e5ddc6b8-c0e3-4ddb-9ddb-0271bbb0b53a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FixedIncomeFundsMember_bc9713f2-17d1-43fd-a046-7217939dbb87" xlink:to="loc_us-gaap_MunicipalBondsMember_e5ddc6b8-c0e3-4ddb-9ddb-0271bbb0b53a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestmentsMember_34c7ad6b-bcdf-4d10-a534-cc222bd0c33a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_c3f5d9a4-3454-4310-a366-6818f4a0ba5c" xlink:to="loc_us-gaap_OtherInvestmentsMember_34c7ad6b-bcdf-4d10-a534-cc222bd0c33a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityFundsMember_07b39e71-be2c-422d-a500-9fa1c50027cf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_c3f5d9a4-3454-4310-a366-6818f4a0ba5c" xlink:to="loc_us-gaap_EquityFundsMember_07b39e71-be2c-422d-a500-9fa1c50027cf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MutualFundMember_03bc08f7-ed66-4a2a-a59b-ee9f3b712782" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MutualFundMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityFundsMember_07b39e71-be2c-422d-a500-9fa1c50027cf" xlink:to="loc_us-gaap_MutualFundMember_03bc08f7-ed66-4a2a-a59b-ee9f3b712782" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_bc64d84c-e5d8-4e17-8d91-1019ce23417c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityFundsMember_07b39e71-be2c-422d-a500-9fa1c50027cf" xlink:to="loc_us-gaap_EquitySecuritiesMember_bc64d84c-e5d8-4e17-8d91-1019ce23417c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_af56af7f-9151-498e-9020-de27ae809f6f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_51bb30fe-58cc-4015-bce5-c2e6c6c8cdf6" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_af56af7f-9151-498e-9020-de27ae809f6f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RetirementPlansScheduleofCostsofRetirementPlansDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RetirementPlansScheduleofCostsofRetirementPlansDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RetirementPlansScheduleofCostsofRetirementPlansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_aa602322-4679-4cfa-8c42-5242fa3fd6e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedContributionPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanTable_ce449b58-c024-4d6f-86ab-374fab2cf06c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedContributionPlanTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_aa602322-4679-4cfa-8c42-5242fa3fd6e5" xlink:to="loc_us-gaap_DefinedContributionPlanTable_ce449b58-c024-4d6f-86ab-374fab2cf06c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_4ecacc0d-c27f-4188-96c3-9f02faf84ba4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DefinedContributionPlanTable_ce449b58-c024-4d6f-86ab-374fab2cf06c" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_4ecacc0d-c27f-4188-96c3-9f02faf84ba4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_4ecacc0d-c27f-4188-96c3-9f02faf84ba4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_4ecacc0d-c27f-4188-96c3-9f02faf84ba4" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_4ecacc0d-c27f-4188-96c3-9f02faf84ba4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_939935c0-46a6-4249-a829-e227ca3d3782" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_4ecacc0d-c27f-4188-96c3-9f02faf84ba4" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_939935c0-46a6-4249-a829-e227ca3d3782" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_c4484321-0fee-4cc2-bf1c-c8c212583235" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_939935c0-46a6-4249-a829-e227ca3d3782" xlink:to="loc_us-gaap_DomesticPlanMember_c4484321-0fee-4cc2-bf1c-c8c212583235" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_df5a886b-f67f-4712-bd0e-a7045618e6c4" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_c4484321-0fee-4cc2-bf1c-c8c212583235" xlink:to="loc_country_US_df5a886b-f67f-4712-bd0e-a7045618e6c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_ec7c9de6-4ed9-4ca4-97fd-43da5f3a4bca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_939935c0-46a6-4249-a829-e227ca3d3782" xlink:to="loc_us-gaap_ForeignPlanMember_ec7c9de6-4ed9-4ca4-97fd-43da5f3a4bca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_b755f9f1-1144-4722-b9d9-c481d650b954" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_ec7c9de6-4ed9-4ca4-97fd-43da5f3a4bca" xlink:to="loc_country_CA_b755f9f1-1144-4722-b9d9-c481d650b954" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_df23c218-cac0-4772-b646-5d315d2ec1cf" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_ec7c9de6-4ed9-4ca4-97fd-43da5f3a4bca" xlink:to="loc_country_GB_df23c218-cac0-4772-b646-5d315d2ec1cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanCostRecognized_5801db84-6fb5-470f-b81a-7ef96da72060" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedContributionPlanCostRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_aa602322-4679-4cfa-8c42-5242fa3fd6e5" xlink:to="loc_us-gaap_DefinedContributionPlanCostRecognized_5801db84-6fb5-470f-b81a-7ef96da72060" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/CommitmentsandContingentLiabilitiesNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#CommitmentsandContingentLiabilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/CommitmentsandContingentLiabilitiesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProductLiabilityContingencyLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductLiabilityContingencyTable_00f51436-ea8b-4682-9c66-6d42773189f9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProductLiabilityContingencyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_us-gaap_ProductLiabilityContingencyTable_00f51436-ea8b-4682-9c66-6d42773189f9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_7e885da8-09bd-4d7d-9d00-9646f887a791" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ProductLiabilityContingencyTable_00f51436-ea8b-4682-9c66-6d42773189f9" xlink:to="loc_srt_MajorCustomersAxis_7e885da8-09bd-4d7d-9d00-9646f887a791" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_7e885da8-09bd-4d7d-9d00-9646f887a791_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_7e885da8-09bd-4d7d-9d00-9646f887a791" xlink:to="loc_srt_NameOfMajorCustomerDomain_7e885da8-09bd-4d7d-9d00-9646f887a791_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_f5ee42a6-02ee-4e0d-93ab-8c8f2471621b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_7e885da8-09bd-4d7d-9d00-9646f887a791" xlink:to="loc_srt_NameOfMajorCustomerDomain_f5ee42a6-02ee-4e0d-93ab-8c8f2471621b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_UPRRMember_b3045c73-7863-4895-8e49-b7bade00e9ab" xlink:href="fstr-20241231.xsd#fstr_UPRRMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_f5ee42a6-02ee-4e0d-93ab-8c8f2471621b" xlink:to="loc_fstr_UPRRMember_b3045c73-7863-4895-8e49-b7bade00e9ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_577b151d-7c78-47bd-8b38-02f0993585e6" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ProductLiabilityContingencyTable_00f51436-ea8b-4682-9c66-6d42773189f9" xlink:to="loc_dei_LegalEntityAxis_577b151d-7c78-47bd-8b38-02f0993585e6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_577b151d-7c78-47bd-8b38-02f0993585e6_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_577b151d-7c78-47bd-8b38-02f0993585e6" xlink:to="loc_dei_EntityDomain_577b151d-7c78-47bd-8b38-02f0993585e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_5e5c0ec3-708e-4058-94c8-0aa6acdd84cc" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_577b151d-7c78-47bd-8b38-02f0993585e6" xlink:to="loc_dei_EntityDomain_5e5c0ec3-708e-4058-94c8-0aa6acdd84cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_UPRRMember_56467506-7c69-48f5-9604-bd9a6ce475e4" xlink:href="fstr-20241231.xsd#fstr_UPRRMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_5e5c0ec3-708e-4058-94c8-0aa6acdd84cc" xlink:to="loc_fstr_UPRRMember_56467506-7c69-48f5-9604-bd9a6ce475e4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_d66ac6e7-8f51-4367-b4ca-e2cc7f83bdb3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LitigationSettlementAmountAwardedToOtherParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_d66ac6e7-8f51-4367-b4ca-e2cc7f83bdb3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LitigationSettlementAmountAwardedToOtherPartyCurrent_198fe800-1d69-4a48-ab8e-842f2e3c4432" xlink:href="fstr-20241231.xsd#fstr_LitigationSettlementAmountAwardedToOtherPartyCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_LitigationSettlementAmountAwardedToOtherPartyCurrent_198fe800-1d69-4a48-ab8e-842f2e3c4432" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LitigationSettlementAmountAwardedToOtherPartyNonCurrent_29cccb23-0041-4a87-b14c-c7883f859f36" xlink:href="fstr-20241231.xsd#fstr_LitigationSettlementAmountAwardedToOtherPartyNonCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_LitigationSettlementAmountAwardedToOtherPartyNonCurrent_29cccb23-0041-4a87-b14c-c7883f859f36" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LitigationSettlementInstallmentPaymentTerm_0a5faa39-c501-497d-af24-07ec579ce341" xlink:href="fstr-20241231.xsd#fstr_LitigationSettlementInstallmentPaymentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_LitigationSettlementInstallmentPaymentTerm_0a5faa39-c501-497d-af24-07ec579ce341" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentAmount_dca4fdd6-d32c-4d06-a463-8f4b5c956a80" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermPurchaseCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentAmount_dca4fdd6-d32c-4d06-a463-8f4b5c956a80" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_LongtermPurchaseCommitmentAnnualCommitmentAmount_895613e2-3e3c-4c8d-98aa-ed7809d2eb4d" xlink:href="fstr-20241231.xsd#fstr_LongtermPurchaseCommitmentAnnualCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_LongtermPurchaseCommitmentAnnualCommitmentAmount_895613e2-3e3c-4c8d-98aa-ed7809d2eb4d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_StandardProductWarrantyAccrualReclassification_518ea5de-1d43-4980-87a3-8c0009c1234a" xlink:href="fstr-20241231.xsd#fstr_StandardProductWarrantyAccrualReclassification"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_StandardProductWarrantyAccrualReclassification_518ea5de-1d43-4980-87a3-8c0009c1234a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LegalFees_41b6f0d0-4e5f-4f89-acf6-86acfd05b0e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LegalFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_us-gaap_LegalFees_41b6f0d0-4e5f-4f89-acf6-86acfd05b0e6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_AccruedLegalSettlementCurrent_9357c2a9-3dc7-479e-8f80-04a31ce5201c" xlink:href="fstr-20241231.xsd#fstr_AccruedLegalSettlementCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_AccruedLegalSettlementCurrent_9357c2a9-3dc7-479e-8f80-04a31ce5201c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_NumberOfCompaniesReceivingGeneralNoticeLetter_9fbcb45c-273c-4126-94e5-1b2e8db737f2" xlink:href="fstr-20241231.xsd#fstr_NumberOfCompaniesReceivingGeneralNoticeLetter"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_NumberOfCompaniesReceivingGeneralNoticeLetter_9fbcb45c-273c-4126-94e5-1b2e8db737f2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_PresentValueOfProposedRemedialWork_60165740-546f-4e74-894a-4572e30a00a4" xlink:href="fstr-20241231.xsd#fstr_PresentValueOfProposedRemedialWork"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_PresentValueOfProposedRemedialWork_60165740-546f-4e74-894a-4572e30a00a4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_UndiscoveredRemedialWork_4752af8f-2fe6-43a2-9774-f83fbea1e844" xlink:href="fstr-20241231.xsd#fstr_UndiscoveredRemedialWork"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_UndiscoveredRemedialWork_4752af8f-2fe6-43a2-9774-f83fbea1e844" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RemedialWorkAnticipatedCleanUpPeriod_7dfcafda-5e2d-4492-96c6-99bb520e9d6a" xlink:href="fstr-20241231.xsd#fstr_RemedialWorkAnticipatedCleanUpPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_RemedialWorkAnticipatedCleanUpPeriod_7dfcafda-5e2d-4492-96c6-99bb520e9d6a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_NumberOfPotentiallyResponsiblePartiesIncludedInAgreement_6309923c-1730-4c17-9374-6c0de4616280" xlink:href="fstr-20241231.xsd#fstr_NumberOfPotentiallyResponsiblePartiesIncludedInAgreement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_NumberOfPotentiallyResponsiblePartiesIncludedInAgreement_6309923c-1730-4c17-9374-6c0de4616280" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_NumberOfPartiesRequiredToPerformRemedialWork_91a4ac43-7211-4e84-8923-d862444e90f9" xlink:href="fstr-20241231.xsd#fstr_NumberOfPartiesRequiredToPerformRemedialWork"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_NumberOfPartiesRequiredToPerformRemedialWork_91a4ac43-7211-4e84-8923-d862444e90f9" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_EnvironmentalLossContingencyStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_0f1d88da-9ba3-49d3-b59a-7b568b61cec9" xlink:href="fstr-20241231.xsd#fstr_EnvironmentalLossContingencyStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_fstr_EnvironmentalLossContingencyStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_0f1d88da-9ba3-49d3-b59a-7b568b61cec9" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_9f80864a-a3d7-43ac-8484-bade57edb00e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccrualForEnvironmentalLossContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingencies_9f80864a-a3d7-43ac-8484-bade57edb00e" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesIncreaseDecreaseForRevisionInEstimates_ab2e7ff2-6525-4ea6-89cc-c01d75e2faec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesIncreaseDecreaseForRevisionInEstimates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductLiabilityContingencyLineItems_62b39f26-1407-4fad-b23e-d0bd51808b57" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesIncreaseDecreaseForRevisionInEstimates_ab2e7ff2-6525-4ea6-89cc-c01d75e2faec" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/OtherExpenseDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#OtherExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/OtherExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_86889bd1-6abe-43df-a60d-d60000a25705" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_86889bd1-6abe-43df-a60d-d60000a25705" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_835ba0fa-b9f9-45c5-a23a-af5ab6becdd5" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_86889bd1-6abe-43df-a60d-d60000a25705" xlink:to="loc_srt_ProductOrServiceAxis_835ba0fa-b9f9-45c5-a23a-af5ab6becdd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_835ba0fa-b9f9-45c5-a23a-af5ab6becdd5_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_835ba0fa-b9f9-45c5-a23a-af5ab6becdd5" xlink:to="loc_srt_ProductsAndServicesDomain_835ba0fa-b9f9-45c5-a23a-af5ab6becdd5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_323f8883-a683-42e6-96c7-275bf331ca63" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_835ba0fa-b9f9-45c5-a23a-af5ab6becdd5" xlink:to="loc_srt_ProductsAndServicesDomain_323f8883-a683-42e6-96c7-275bf331ca63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_ChemtecEnergyServicesLLCMember_c7d0032b-ef02-464d-97a5-35fcf8a52047" xlink:href="fstr-20241231.xsd#fstr_ChemtecEnergyServicesLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_323f8883-a683-42e6-96c7-275bf331ca63" xlink:to="loc_fstr_ChemtecEnergyServicesLLCMember_c7d0032b-ef02-464d-97a5-35fcf8a52047" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_ConcreteTiesMember_66aaf63f-4d68-4516-b79d-101f7cdd792e" xlink:href="fstr-20241231.xsd#fstr_ConcreteTiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_323f8883-a683-42e6-96c7-275bf331ca63" xlink:to="loc_fstr_ConcreteTiesMember_66aaf63f-4d68-4516-b79d-101f7cdd792e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_2c3046cf-8e33-4fb0-a14d-c29d85d35815" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_2c3046cf-8e33-4fb0-a14d-c29d85d35815" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnDispositionOfAssets1_44c839db-f890-41f3-b3f0-91ad74e06457" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnDispositionOfAssets1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_us-gaap_GainLossOnDispositionOfAssets1_44c839db-f890-41f3-b3f0-91ad74e06457" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InsuranceProceeds_dc99f06e-1b4e-4d33-adaa-b6f5374a8107" xlink:href="fstr-20241231.xsd#fstr_InsuranceProceeds"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_fstr_InsuranceProceeds_dc99f06e-1b4e-4d33-adaa-b6f5374a8107" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_4d1534e5-1525-4149-a44a-f7b2b3eb26a5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_4d1534e5-1525-4149-a44a-f7b2b3eb26a5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIncome_3a211e35-490e-4612-b6b0-b929506aa6e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_us-gaap_OtherIncome_3a211e35-490e-4612-b6b0-b929506aa6e5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_2a390c68-8a48-4f0c-8550-562660c9dbd3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_2a390c68-8a48-4f0c-8550-562660c9dbd3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_14b140a5-1d22-4f3c-af33-ed0b6c6ee264" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_93b9d4f3-0579-4789-ad28-4278500de599" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_14b140a5-1d22-4f3c-af33-ed0b6c6ee264" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RestructuringCostsScheduleofRestructuringandRelatedCostsbySegmentDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RestructuringCostsScheduleofRestructuringandRelatedCostsbySegmentDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RestructuringCostsScheduleofRestructuringandRelatedCostsbySegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_e2f2e305-51e8-4044-9a8d-cd715e752fda" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_564f4776-1093-4140-bd81-7c52479e710a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e2f2e305-51e8-4044-9a8d-cd715e752fda" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_564f4776-1093-4140-bd81-7c52479e710a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_42883bab-e932-4fea-8970-bffe4e64bca5" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_564f4776-1093-4140-bd81-7c52479e710a" xlink:to="loc_srt_ConsolidationItemsAxis_42883bab-e932-4fea-8970-bffe4e64bca5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_42883bab-e932-4fea-8970-bffe4e64bca5_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_42883bab-e932-4fea-8970-bffe4e64bca5" xlink:to="loc_srt_ConsolidationItemsDomain_42883bab-e932-4fea-8970-bffe4e64bca5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_45fc8f93-e7aa-4399-95b2-e91e91905f9f" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_42883bab-e932-4fea-8970-bffe4e64bca5" xlink:to="loc_srt_ConsolidationItemsDomain_45fc8f93-e7aa-4399-95b2-e91e91905f9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_51d31e16-d204-4a28-b5a8-8eb2c8ca7fba" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_45fc8f93-e7aa-4399-95b2-e91e91905f9f" xlink:to="loc_us-gaap_OperatingSegmentsMember_51d31e16-d204-4a28-b5a8-8eb2c8ca7fba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_4b0c7f3f-d368-4004-8f6f-3e84a0a3339b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_45fc8f93-e7aa-4399-95b2-e91e91905f9f" xlink:to="loc_us-gaap_CorporateNonSegmentMember_4b0c7f3f-d368-4004-8f6f-3e84a0a3339b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_80d6f9cd-baca-4792-8f4f-b2dc4c45563d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_564f4776-1093-4140-bd81-7c52479e710a" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_80d6f9cd-baca-4792-8f4f-b2dc4c45563d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_80d6f9cd-baca-4792-8f4f-b2dc4c45563d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_80d6f9cd-baca-4792-8f4f-b2dc4c45563d" xlink:to="loc_us-gaap_SegmentDomain_80d6f9cd-baca-4792-8f4f-b2dc4c45563d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_5ec05086-ba3d-48c9-afb8-3d7fc28f633b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_80d6f9cd-baca-4792-8f4f-b2dc4c45563d" xlink:to="loc_us-gaap_SegmentDomain_5ec05086-ba3d-48c9-afb8-3d7fc28f633b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_RailTechnologiesAndServicesSegmentMember_7dc79c63-bf20-422b-b12d-3e5a5ff0391c" xlink:href="fstr-20241231.xsd#fstr_RailTechnologiesAndServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_5ec05086-ba3d-48c9-afb8-3d7fc28f633b" xlink:to="loc_fstr_RailTechnologiesAndServicesSegmentMember_7dc79c63-bf20-422b-b12d-3e5a5ff0391c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fstr_InfrastructureSolutionsSegmentMember_896336b3-8501-4f2d-95e3-459f8d1d328f" xlink:href="fstr-20241231.xsd#fstr_InfrastructureSolutionsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_5ec05086-ba3d-48c9-afb8-3d7fc28f633b" xlink:to="loc_fstr_InfrastructureSolutionsSegmentMember_896336b3-8501-4f2d-95e3-459f8d1d328f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_ad533b7f-24f4-445a-8915-8ec69009e1e8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_e2f2e305-51e8-4044-9a8d-cd715e752fda" xlink:to="loc_us-gaap_RestructuringCharges_ad533b7f-24f4-445a-8915-8ec69009e1e8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/RestructuringCostsNarrativeDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#RestructuringCostsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/RestructuringCostsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_2a229c8c-ebf7-4c16-939f-60cbee4811ba" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_47157dbd-e38a-4b34-a006-bb57ff972243" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_2a229c8c-ebf7-4c16-939f-60cbee4811ba" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_47157dbd-e38a-4b34-a006-bb57ff972243" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_d7cce67e-497c-42b1-801b-7b73a5eb4d27" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_47157dbd-e38a-4b34-a006-bb57ff972243" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_d7cce67e-497c-42b1-801b-7b73a5eb4d27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_d7cce67e-497c-42b1-801b-7b73a5eb4d27_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_d7cce67e-497c-42b1-801b-7b73a5eb4d27" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_d7cce67e-497c-42b1-801b-7b73a5eb4d27_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_cd5c0b3e-1a0e-408b-8e0d-6045bb6aa193" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_d7cce67e-497c-42b1-801b-7b73a5eb4d27" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_cd5c0b3e-1a0e-408b-8e0d-6045bb6aa193" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_17fd186b-a1df-46cc-822d-aefd0bdaf538" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_cd5c0b3e-1a0e-408b-8e0d-6045bb6aa193" xlink:to="loc_us-gaap_CostOfSalesMember_17fd186b-a1df-46cc-822d-aefd0bdaf538" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_c9eed876-f272-469a-aef2-da61ee1b0021" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_cd5c0b3e-1a0e-408b-8e0d-6045bb6aa193" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_c9eed876-f272-469a-aef2-da61ee1b0021" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_ab80b28e-f33f-4a3b-8b54-2a4941bc6cb6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_2a229c8c-ebf7-4c16-939f-60cbee4811ba" xlink:to="loc_us-gaap_RestructuringCharges_ab80b28e-f33f-4a3b-8b54-2a4941bc6cb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_be2b7eb6-cc5a-4962-bdec-0d8b20c28318" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_2a229c8c-ebf7-4c16-939f-60cbee4811ba" xlink:to="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_be2b7eb6-cc5a-4962-bdec-0d8b20c28318" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_20c24945-bb2d-449c-8bd8-992775f332a6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_b490a6cc-5809-4396-a3f3-62d067ec1241" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_20c24945-bb2d-449c-8bd8-992775f332a6" xlink:to="loc_us-gaap_SubsequentEventTable_b490a6cc-5809-4396-a3f3-62d067ec1241" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_e9ea58c3-0407-4f2d-880c-11d3d3c4c576" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_b490a6cc-5809-4396-a3f3-62d067ec1241" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_e9ea58c3-0407-4f2d-880c-11d3d3c4c576" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_e9ea58c3-0407-4f2d-880c-11d3d3c4c576_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_e9ea58c3-0407-4f2d-880c-11d3d3c4c576" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_e9ea58c3-0407-4f2d-880c-11d3d3c4c576_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_8f9cbcd8-5480-42b5-b407-16d4cfc74fbf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_e9ea58c3-0407-4f2d-880c-11d3d3c4c576" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_8f9cbcd8-5480-42b5-b407-16d4cfc74fbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_e44b1ec0-93ce-47c9-bb3b-228842cd998d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_8f9cbcd8-5480-42b5-b407-16d4cfc74fbf" xlink:to="loc_us-gaap_SubsequentEventMember_e44b1ec0-93ce-47c9-bb3b-228842cd998d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_7530079b-1a97-464d-8324-9c4b6b1e3a8d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_20c24945-bb2d-449c-8bd8-992775f332a6" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_7530079b-1a97-464d-8324-9c4b6b1e3a8d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.lbfoster.com/role/ScheduleIIValuationandQualifyingAccountsDetails" xlink:type="simple" xlink:href="fstr-20241231.xsd#ScheduleIIValuationandQualifyingAccountsDetails"/>
  <link:definitionLink xlink:role="http://www.lbfoster.com/role/ScheduleIIValuationandQualifyingAccountsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_68b77163-de2b-4599-a3ed-18a6722c3a3b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ValuationAndQualifyingAccountsDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ValuationAndQualifyingAccountsDisclosureTable_987c0ed7-fce0-4914-b27b-aff0c6485fb5" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ValuationAndQualifyingAccountsDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_68b77163-de2b-4599-a3ed-18a6722c3a3b" xlink:to="loc_srt_ValuationAndQualifyingAccountsDisclosureTable_987c0ed7-fce0-4914-b27b-aff0c6485fb5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_f05de2fc-db35-4fda-b388-0fb9d461d614" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureTable_987c0ed7-fce0-4914-b27b-aff0c6485fb5" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_f05de2fc-db35-4fda-b388-0fb9d461d614" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_f05de2fc-db35-4fda-b388-0fb9d461d614_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_f05de2fc-db35-4fda-b388-0fb9d461d614" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_f05de2fc-db35-4fda-b388-0fb9d461d614_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_bb4cd6ac-0efa-4256-bd6a-3f827a5a833b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_f05de2fc-db35-4fda-b388-0fb9d461d614" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_bb4cd6ac-0efa-4256-bd6a-3f827a5a833b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForCreditLossMember_b537158e-51ed-46f2-becf-2528007ea553" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllowanceForCreditLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_bb4cd6ac-0efa-4256-bd6a-3f827a5a833b" xlink:to="loc_us-gaap_AllowanceForCreditLossMember_b537158e-51ed-46f2-becf-2528007ea553" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceOfDeferredTaxAssetsMember_ccdedebc-968e-432a-a1cd-8c0a21c9025f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowanceOfDeferredTaxAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_bb4cd6ac-0efa-4256-bd6a-3f827a5a833b" xlink:to="loc_us-gaap_ValuationAllowanceOfDeferredTaxAssetsMember_ccdedebc-968e-432a-a1cd-8c0a21c9025f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_8e2985d7-bbb6-4bb0-9513-7cf61bc64103" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MovementInValuationAllowancesAndReservesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_68b77163-de2b-4599-a3ed-18a6722c3a3b" xlink:to="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_8e2985d7-bbb6-4bb0-9513-7cf61bc64103" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_6127b0a3-80d0-44c4-b8a6-e2d9a689dc64" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_8e2985d7-bbb6-4bb0-9513-7cf61bc64103" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesBalance_6127b0a3-80d0-44c4-b8a6-e2d9a689dc64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense_639fce00-2a72-469e-b2d0-34b0bb4bb589" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_8e2985d7-bbb6-4bb0-9513-7cf61bc64103" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense_639fce00-2a72-469e-b2d0-34b0bb4bb589" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDeductions_0040ff09-6c14-4d61-bb4f-327f7ba997c7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesDeductions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_8e2985d7-bbb6-4bb0-9513-7cf61bc64103" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDeductions_0040ff09-6c14-4d61-bb4f-327f7ba997c7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_75277a00-b18c-4d0f-ac42-9b12540b05f5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesBalance"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-roles-2024.xsd#eedm"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:type="arc" order="41"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent" xlink:type="arc" order="42"/>
  </link:definitionLink>
</link:linkbase>
