diff --git a/apps/edr-freight-web/backoffice/src/components/trainBuilder/BuildTrainModal.tsx b/apps/edr-freight-web/backoffice/src/components/trainBuilder/BuildTrainModal.tsx
index 7f60cabe9..49dd19bcc 100644
--- a/apps/edr-freight-web/backoffice/src/components/trainBuilder/BuildTrainModal.tsx
+++ b/apps/edr-freight-web/backoffice/src/components/trainBuilder/BuildTrainModal.tsx
@@ -16,6 +16,7 @@ import { useEffect, useState } from "react";
 import { api } from "@/services/api";
 import type { TrainComposition } from "@/services/trainBuilder.service";
 import { useToast } from "@/hooks/use-toast";
+import { EXPORT_TRAIN_OPTIONS, importRunFor } from "@/constants/trainRuns";
 
 const parseError = (error: unknown, fallback: string) => {
   if (isAxiosError(error)) {
@@ -26,10 +27,6 @@ const parseError = (error: unknown, fallback: string) => {
   return fallback;
 };
 
-// Run-number parity carries the trade direction: odd = export, even = import.
-const isOddNumber = (value: string) => /^\d*[13579]$/.test(value.trim());
-const isEvenNumber = (value: string) => /^\d*[02468]$/.test(value.trim());
-
 /**
  * Step one of the Train Builder: pick the yard it is being assembled in and
  * couple at least two locomotives from that yard. The train code is assigned by
@@ -59,6 +56,12 @@ export default function BuildTrainModal({ opened, onClose, onBuilt }: BuildTrain
     setLocomotiveIds([]);
   }, [yardId]);
 
+  // The import run is fixed by the export run, so it tracks it rather than
+  // being entered by hand (and clears back to empty when the export is cleared).
+  useEffect(() => {
+    setImportTrainNumber(importRunFor(exportTrainNumber));
+  }, [exportTrainNumber]);
+
   useEffect(() => {
     if (!opened) {
       setExportTrainNumber("");
@@ -78,9 +81,11 @@ export default function BuildTrainModal({ opened, onClose, onBuilt }: BuildTrain
       });
       return;
     }
-    if (!isOddNumber(exportTrainNumber) || !isEvenNumber(importTrainNumber)) {
+    // Both numbers come from the fixed run pairs, so parity cannot be wrong —
+    // only "nothing picked" is reachable here.
+    if (!exportTrainNumber || !importTrainNumber) {
       toast({
-        title: "Enter both run numbers — export must be odd (e.g. 8001), import even (e.g. 8002)",
+        title: "Pick an export train number (e.g. 8001) — the import run follows it",
         variant: "destructive",
       });
       return;
@@ -133,31 +138,24 @@ export default function BuildTrainModal({ opened, onClose, onBuilt }: BuildTrain
           maxLength={100}
         />
         <Group grow>
-          <TextInput
+          <Select
             label="Export train number"
             description="Odd — Ethiopia → Djibouti runs"
             placeholder="e.g. 8001"
-            value={exportTrainNumber}
-            onChange={(e) => setExportTrainNumber(e.currentTarget.value)}
-            maxLength={20}
-            error={
-              exportTrainNumber && !isOddNumber(exportTrainNumber)
-                ? "Must be numeric and odd"
-                : undefined
-            }
+            data={EXPORT_TRAIN_OPTIONS}
+            value={exportTrainNumber || null}
+            onChange={(value) => setExportTrainNumber(value ?? "")}
+            searchable
+            clearable
           />
+          {/* Fixed by the export run — derived, never typed. */}
           <TextInput
             label="Import train number"
             description="Even — Djibouti → Ethiopia runs"
             placeholder="e.g. 8002"
             value={importTrainNumber}
-            onChange={(e) => setImportTrainNumber(e.currentTarget.value)}
-            maxLength={20}
-            error={
-              importTrainNumber && !isEvenNumber(importTrainNumber)
-                ? "Must be numeric and even"
-                : undefined
-            }
+            readOnly
+            variant="filled"
           />
         </Group>
         <Select
diff --git a/apps/edr-freight-web/backoffice/src/constants/trainRuns.ts b/apps/edr-freight-web/backoffice/src/constants/trainRuns.ts
new file mode 100644
index 000000000..10a01f577
--- /dev/null
+++ b/apps/edr-freight-web/backoffice/src/constants/trainRuns.ts
@@ -0,0 +1,35 @@
+/**
+ * EDR run-number pairs, keyed by the odd EXPORT run (Ethiopia → Djibouti). The
+ * even IMPORT run (Djibouti → Ethiopia) is fixed by the export run, so choosing
+ * an export number fully determines the import one.
+ *
+ * Run numbers are always 4 digits (8401, never 84001). Pairs are listed out
+ * rather than computed from the 8001/+100/+1 pattern, so a run that ever breaks
+ * the convention stays correct here.
+ *
+ * Mirrors RUN_WAGONS/IMPORT_RUN in the API's SeedWagonRunNumbers migration —
+ * keep the two in sync when runs are added or retired.
+ */
+export const TRAIN_RUN_PAIRS: Record<string, string> = {
+  "8001": "8002",
+  "8101": "8102",
+  "8201": "8202",
+  "8301": "8302",
+  "8401": "8402",
+  "8501": "8502",
+  "8601": "8602",
+  "8701": "8702",
+  "8801": "8802",
+  "8901": "8902",
+  "9001": "9002",
+};
+
+/** Selectable export runs, in run order. */
+export const EXPORT_TRAIN_OPTIONS = Object.keys(TRAIN_RUN_PAIRS).map((run) => ({
+  label: run,
+  value: run,
+}));
+
+/** The import run implied by an export run; empty string when unset/unknown. */
+export const importRunFor = (exportRun: unknown): string =>
+  TRAIN_RUN_PAIRS[String(exportRun ?? "")] ?? "";
diff --git a/apps/edr-freight-web/backoffice/src/pages/fleet/config/resources.ts b/apps/edr-freight-web/backoffice/src/pages/fleet/config/resources.ts
index 83928e146..793537dd9 100644
--- a/apps/edr-freight-web/backoffice/src/pages/fleet/config/resources.ts
+++ b/apps/edr-freight-web/backoffice/src/pages/fleet/config/resources.ts
@@ -1,5 +1,6 @@
 import { Freight } from "@edr/types";
 import type { ColumnFormat, FormFieldDef } from "@/pages/ruleEngine/config/resources";
+import { EXPORT_TRAIN_OPTIONS, importRunFor } from "@/constants/trainRuns";
 import { vehiclesConfig, VEHICLE_TYPE_OPTIONS, FUEL_TYPE_OPTIONS, VEHICLE_STATUS_OPTIONS } from "./vehicles";
 import { driversConfig, DRIVER_STATUS_OPTIONS } from "./drivers";
 
@@ -130,33 +131,6 @@ const WAGON_STATUS_OPTIONS = [
   { label: "Detained", value: Freight.WagonStatus.Detained },
 ];
 
-/**
- * EDR run-number pairs, keyed by the odd EXPORT run (Ethiopia → Djibouti). The
- * even IMPORT run (Djibouti → Ethiopia) is fixed by the export run, so choosing
- * an export number fully determines the import one.
- *
- * Run numbers are always 4 digits (8401, never 84001). Pairs are listed out
- * rather than computed from the 8001/+100/+1 pattern, so a run that ever breaks
- * the convention stays correct here.
- */
-const TRAIN_RUN_PAIRS: Record<string, string> = {
-  "8001": "8002",
-  "8101": "8102",
-  "8201": "8202",
-  "8301": "8302",
-  "8401": "8402",
-  "8501": "8502",
-  "8601": "8602",
-  "8701": "8702",
-  "8801": "8802",
-  "8901": "8902",
-  "9001": "9002",
-};
-
-const EXPORT_TRAIN_OPTIONS = Object.keys(TRAIN_RUN_PAIRS).map((run) => ({
-  label: run,
-  value: run,
-}));
 
 
 
@@ -342,8 +316,7 @@ export const FLEET_RESOURCES: FleetResourceConfig[] = [
         type: "text",
         description: "Even — Djibouti → Ethiopia runs",
         placeholder: "e.g. 8002",
-        derivedValue: (values) =>
-          TRAIN_RUN_PAIRS[String(values.exportTrainNumber ?? "")] ?? "",
+        derivedValue: (values) => importRunFor(values.exportTrainNumber),
         // Follows the export run to NULL when that is cleared.
         clearable: true,
       },