diff --git a/apps/edr-freight-api/src/migrations/2260000000000-SeedEdrWagonFleetErNumbering.ts b/apps/edr-freight-api/src/migrations/2260000000000-SeedEdrWagonFleetErNumbering.ts
index fdfcf8be5..717a48217 100644
--- a/apps/edr-freight-api/src/migrations/2260000000000-SeedEdrWagonFleetErNumbering.ts
+++ b/apps/edr-freight-api/src/migrations/2260000000000-SeedEdrWagonFleetErNumbering.ts
@@ -44,6 +44,14 @@ export class SeedEdrWagonFleetErNumbering2260000000000 implements MigrationInter
     // train_set_wagons null their link, wagon_movements cascade.
     await queryRunner.query(`DELETE FROM freight.wagons;`);
 
+    // Wagon.wagonNumber declares `unique: true`, but some environments never got
+    // the constraint. Repair it here — the table is empty at this point, so the
+    // index build cannot fail on pre-existing duplicates.
+    await queryRunner.query(`
+      CREATE UNIQUE INDEX IF NOT EXISTS wagons_wagon_number_key
+        ON freight.wagons (wagon_number);
+    `);
+
     for (const row of FLEET) {
       if (row.end - row.start + 1 !== row.count) {
         throw new Error(`wagon_range_mismatch:${row.code}`);
@@ -59,7 +67,9 @@ export class SeedEdrWagonFleetErNumbering2260000000000 implements MigrationInter
       }
 
       // generate_series builds the range server-side — one round trip per type
-      // instead of 1100 individual INSERTs.
+      // instead of 1100 individual INSERTs. No ON CONFLICT clause: every wagon
+      // was deleted above, so a plain INSERT cannot collide, and the clause would
+      // otherwise hard-require a unique index this table lacks on some envs.
       await queryRunner.query(
         `
           INSERT INTO freight.wagons (
@@ -83,12 +93,7 @@ export class SeedEdrWagonFleetErNumbering2260000000000 implements MigrationInter
             NULL,
             NULL,
             NULL
-          FROM generate_series($2::int, $3::int) AS seq
-          ON CONFLICT (wagon_number) DO UPDATE SET
-            wagon_type_id   = EXCLUDED.wagon_type_id,
-            status          = EXCLUDED.status,
-            current_yard_id = EXCLUDED.current_yard_id,
-            updated_at      = now();
+          FROM generate_series($2::int, $3::int) AS seq;
         `,
         [typeRecord.id, row.start, row.end],
       );