feat(train-sets): implement multi-locomotive support for train sets

- Added TrainSetLocomotive entity to link multiple locomotives to a train set.
- Updated TrainSet entity to include a OneToMany relationship with TrainSetLocomotive.
- Modified the train scheduling logic to require at least two locomotives for a train set.
- Enhanced the UI components to support selecting multiple locomotives.
- Introduced new permissions for viewing customs clearance.
- Updated migrations to create the train_set_locomotives table and backfill existing data.
- Implemented utility functions for managing train numbers based on cargo type and direction.
- Added tests for train number utilities to ensure correct functionality.

apps/edr-freight-api/src/modules/train-scheduling/dto/create-container-train-schedule.dto.ts

@@ -1,6 +1,15 @@
 import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
 import { Type } from 'class-transformer';
-import { IsDateString, IsInt, IsNumber, IsOptional, IsUUID, Min } from 'class-validator';
+import {
+  ArrayMinSize,
+  IsArray,
+  IsDateString,
+  IsInt,
+  IsNumber,
+  IsOptional,
+  IsUUID,
+  Min,
+} from 'class-validator';
 
 export class CreateContainerTrainScheduleDto {
   @ApiProperty({ format: 'uuid' })
@@ -11,9 +20,15 @@ export class CreateContainerTrainScheduleDto {
   @IsDateString()
   scheduleDate!: string;
 
-  @ApiProperty({ format: 'uuid' })
-  @IsUUID()
-  locomotiveId!: string;
+  @ApiProperty({
+    type: [String],
+    format: 'uuid',
+    description: 'Locomotives pulling the train (minimum 2 — front and back)',
+  })
+  @IsArray()
+  @ArrayMinSize(2, { message: 'A train must be pulled by at least two locomotives' })
+  @IsUUID('all', { each: true })
+  locomotiveIds!: string[];
 
   @ApiPropertyOptional({ description: 'Maximum total booking weight allowed on this train' })
   @IsOptional()

apps/edr-freight-api/src/modules/train-scheduling/train-capacity.util.ts

@@ -69,6 +69,25 @@ export function deriveTrainCapacityFromLocomotive(
 export const MAX_FALLBACK_WEIGHT = 3500;
 export const MAX_FALLBACK_LENGTH = 760;
 
+/**
+ * Effective pull limits for a train set with multiple locomotives: the weakest
+ * locomotive caps the train, so take the minimum pull weight and minimum length
+ * across all assigned locomotives. Returns null when no locomotives are given.
+ */
+export function minLocomotiveLimits(
+  locomotives: Array<Pick<LocomotiveLimits, 'maxPullWeightTons' | 'maxTrainLengthMeters'>>,
+): LocomotiveLimits | null {
+  if (!locomotives.length) return null;
+  return {
+    maxPullWeightTons: Math.min(
+      ...locomotives.map((l) => Number(l.maxPullWeightTons) || Infinity),
+    ),
+    maxTrainLengthMeters: Math.min(
+      ...locomotives.map((l) => Number(l.maxTrainLengthMeters) || Infinity),
+    ),
+  };
+}
+
 /** Per-booking train length from wagon count and freight-specific wagon type length. */
 export function bookingTrainLengthMeters(
   freightType: string | null | undefined,

apps/edr-freight-api/src/modules/train-scheduling/train-number.util.spec.ts

@@ -0,0 +1,52 @@
+import {
+  BULK_IMPORT_NUMBERS,
+  CONTAINER_EXPORT_NUMBERS,
+  CONTAINER_IMPORT_NUMBERS,
+  pickLowestFreeNumber,
+  pickTrainNumberPool,
+} from './train-number.util';
+
+describe('train-number.util', () => {
+  describe('pickTrainNumberPool', () => {
+    it('picks container export (odd) when container wagons dominate and direction is EXPORT', () => {
+      const pool = pickTrainNumberPool(5, 2, 'EXPORT');
+      expect(pool.cargo).toBe('CONTAINER');
+      expect(pool.direction).toBe('EXPORT');
+      expect(pool.numbers).toEqual(CONTAINER_EXPORT_NUMBERS);
+    });
+
+    it('picks container import (even) when container wagons dominate and direction is IMPORT', () => {
+      const pool = pickTrainNumberPool(5, 2, 'IMPORT');
+      expect(pool.numbers).toEqual(CONTAINER_IMPORT_NUMBERS);
+    });
+
+    it('picks bulk when bulk wagons dominate', () => {
+      const pool = pickTrainNumberPool(1, 9, 'IMPORT');
+      expect(pool.cargo).toBe('BULK');
+      expect(pool.numbers).toEqual(BULK_IMPORT_NUMBERS);
+    });
+
+    it('treats a tie as container', () => {
+      expect(pickTrainNumberPool(3, 3, 'EXPORT').cargo).toBe('CONTAINER');
+    });
+
+    it('defaults DOMESTIC to the export/odd pool', () => {
+      expect(pickTrainNumberPool(5, 0, 'DOMESTIC').direction).toBe('EXPORT');
+      expect(pickTrainNumberPool(5, 0, null).direction).toBe('EXPORT');
+    });
+  });
+
+  describe('pickLowestFreeNumber', () => {
+    it('returns the lowest unused number', () => {
+      expect(pickLowestFreeNumber(CONTAINER_EXPORT_NUMBERS, ['8001'])).toBe('8101');
+    });
+
+    it('returns the first number when none are used', () => {
+      expect(pickLowestFreeNumber(CONTAINER_EXPORT_NUMBERS, [])).toBe('8001');
+    });
+
+    it('returns null when the pool is exhausted', () => {
+      expect(pickLowestFreeNumber(BULK_IMPORT_NUMBERS, [...BULK_IMPORT_NUMBERS])).toBeNull();
+    });
+  });
+});

apps/edr-freight-api/src/modules/train-scheduling/train-number.util.ts

@@ -0,0 +1,68 @@
+/**
+ * Fixed train-number pools assigned to a train on dispatch.
+ *
+ * The prefix encodes cargo type (8 = container, 1 = bulk) and the parity encodes
+ * trade direction (odd = export, even = import). Numbers are finite and recycle:
+ * a number is "in use" only while its train is DISPATCHED and not yet ARRIVED.
+ */
+
+export const CONTAINER_EXPORT_NUMBERS = [
+  '8001', '8101', '8201', '8301', '8401', '8501', '8601', '8701', '8801', '8901',
+] as const;
+
+export const CONTAINER_IMPORT_NUMBERS = [
+  '8002', '8102', '8202', '8302', '8402', '8502', '8602', '8702', '8802', '8902',
+] as const;
+
+export const BULK_EXPORT_NUMBERS = ['1101', '1103', '1105', '1107'] as const;
+
+export const BULK_IMPORT_NUMBERS = ['1002', '1004', '1006', '1008'] as const;
+
+export type CargoKind = 'CONTAINER' | 'BULK';
+export type PoolDirection = 'IMPORT' | 'EXPORT';
+
+export interface TrainNumberPool {
+  cargo: CargoKind;
+  /** EXPORT = odd numbers, IMPORT = even numbers. */
+  direction: PoolDirection;
+  numbers: readonly string[];
+}
+
+/**
+ * Resolve which fixed pool a train draws from.
+ *
+ * - Cargo: container vs bulk by dominant wagon count; ties resolve to container.
+ * - Direction: EXPORT → odd pool, IMPORT → even pool. DOMESTIC (neither end is
+ *   Djibouti) has no dedicated pool, so it defaults to the export/odd pool.
+ */
+export function pickTrainNumberPool(
+  containerWagons: number,
+  bulkWagons: number,
+  direction: 'IMPORT' | 'EXPORT' | 'DOMESTIC' | null | undefined,
+): TrainNumberPool {
+  const cargo: CargoKind = bulkWagons > containerWagons ? 'BULK' : 'CONTAINER';
+  const poolDirection: PoolDirection = direction === 'IMPORT' ? 'IMPORT' : 'EXPORT';
+
+  const numbers =
+    cargo === 'CONTAINER'
+      ? poolDirection === 'IMPORT'
+        ? CONTAINER_IMPORT_NUMBERS
+        : CONTAINER_EXPORT_NUMBERS
+      : poolDirection === 'IMPORT'
+        ? BULK_IMPORT_NUMBERS
+        : BULK_EXPORT_NUMBERS;
+
+  return { cargo, direction: poolDirection, numbers };
+}
+
+/** Lowest pool number not currently in use, or null when the pool is exhausted. */
+export function pickLowestFreeNumber(
+  pool: readonly string[],
+  usedNumbers: Iterable<string>,
+): string | null {
+  const used = new Set(usedNumbers);
+  for (const number of pool) {
+    if (!used.has(number)) return number;
+  }
+  return null;
+}

apps/edr-freight-api/src/modules/train-scheduling/train-scheduling.module.ts

@@ -7,6 +7,7 @@ import { LocomotivesModule } from '../locomotives/locomotives.module';
 import { RuleEngineModule } from '../rule-engine/rule-engine.module';
 import { Locomotive } from '../locomotives/entities/locomotive.entity';
 import { Route } from '../routes/entities/route.entity';
+import { TrainSetLocomotive } from '../train-sets/entities/train-set-locomotive.entity';
 import { TrainSetWagon } from '../train-sets/entities/train-set-wagon.entity';
 import { TrainSet } from '../train-sets/entities/train-set.entity';
 import { TrainSetsModule } from '../train-sets/train-sets.module';
@@ -30,6 +31,7 @@ import { NotificationsModule } from '../notifications/notifications.module';
       WagonType,
       TrainSet,
       TrainSetWagon,
+      TrainSetLocomotive,
       Route,
       Wagon,
       Container,

apps/edr-freight-api/src/modules/train-scheduling/train-scheduling.service.spec.ts

@@ -389,8 +389,12 @@ describe('TrainSchedulingService', () => {
       isActive: true,
     };
 
+    const locomotive2 = { ...locomotive, id: 'loc-2', code: 'LOC-002' };
     const lockedLocomotiveRepo = {
-      findOne: jest.fn().mockResolvedValue(locomotive),
+      findOne: jest
+        .fn()
+        .mockResolvedValueOnce(locomotive)
+        .mockResolvedValueOnce(locomotive2),
       update: jest.fn().mockResolvedValue(undefined),
     };
     const trainScheduleRepo = {
@@ -401,6 +405,10 @@ describe('TrainSchedulingService', () => {
       create: jest.fn().mockImplementation((value) => value),
       save: jest.fn().mockResolvedValue({ id: 'train-set-1' }),
     };
+    const trainSetLocomotiveRepo = {
+      create: jest.fn().mockImplementation((value) => value),
+      save: jest.fn().mockResolvedValue(undefined),
+    };
     const manager = {
       getRepository: jest.fn((entity: { name?: string }) => {
         switch (entity?.name) {
@@ -410,13 +418,14 @@ describe('TrainSchedulingService', () => {
             return trainScheduleRepo;
           case 'TrainSet':
             return trainSetRepo;
+          case 'TrainSetLocomotive':
+            return trainSetLocomotiveRepo;
           default:
             throw new Error(`Unexpected transaction repository ${entity?.name}`);
         }
       }),
     };
 
-    jest.spyOn(service, 'selectOrValidateLocomotive').mockResolvedValue(locomotive as never);
     dataSource.getRepository.mockImplementation((entity: unknown) => {
       if ((entity as { name?: string })?.name === 'Route') {
         return { findOne: jest.fn().mockResolvedValue(route) };
@@ -437,12 +446,16 @@ describe('TrainSchedulingService', () => {
     const result = await service.createContainerTrainSchedule({
       routeId: 'route-1',
       scheduleDate: '2026-06-20T08:00:00.000Z',
-      locomotiveId: 'loc-1',
+      locomotiveIds: ['loc-1', 'loc-2'],
     });
 
     expect(trainSetRepo.save).toHaveBeenCalled();
     expect(trainScheduleRepo.save).toHaveBeenCalled();
-    expect(lockedLocomotiveRepo.update).toHaveBeenCalledWith('loc-1', { status: 'ASSIGNED' });
+    expect(trainSetLocomotiveRepo.save).toHaveBeenCalled();
+    expect(lockedLocomotiveRepo.update).toHaveBeenCalledWith(
+      { id: expect.objectContaining({ _type: 'in', _value: ['loc-1', 'loc-2'] }) },
+      { status: 'ASSIGNED' },
+    );
     expect(result.id).toBe('schedule-1');
   });
 
@@ -508,7 +521,6 @@ describe('TrainSchedulingService', () => {
       })),
     };
 
-    jest.spyOn(service, 'selectOrValidateLocomotive').mockResolvedValue(locomotive as never);
     dataSource.getRepository.mockImplementation((entity: { name?: string }) => {
       if (entity?.name === 'Route') {
         return {
@@ -531,7 +543,7 @@ describe('TrainSchedulingService', () => {
       service.createContainerTrainSchedule({
         routeId: 'route-1',
         scheduleDate: '2026-06-20T08:00:00.000Z',
-        locomotiveId: 'loc-1',
+        locomotiveIds: ['loc-1', 'loc-2'],
       }),
     ).rejects.toBeInstanceOf(ConflictException);
   });

apps/edr-freight-api/src/modules/train-scheduling/train-scheduling.service.ts

@@ -22,6 +22,7 @@ import { Container } from '../container-management/entities/container.entity';
 import { Locomotive } from '../locomotives/entities/locomotive.entity';
 import { LocomotivesRepository } from '../locomotives/locomotives.repository';
 import { Route } from '../routes/entities/route.entity';
+import { TrainSetLocomotive } from '../train-sets/entities/train-set-locomotive.entity';
 import { TrainSetWagon } from '../train-sets/entities/train-set-wagon.entity';
 import { TrainSet } from '../train-sets/entities/train-set.entity';
 import { TrainScheduleBooking } from '../train-schedules/entities/train-schedule-booking.entity';
@@ -79,8 +80,10 @@ import {
   pickBulkWagonType,
 } from './wagon-type-resolver.util';
 import { deriveScheduleDirection } from './derive-schedule-direction.util';
+import { pickLowestFreeNumber, pickTrainNumberPool } from './train-number.util';
 import {
   deriveTrainCapacityFromLocomotive,
+  minLocomotiveLimits,
   wagonTypeDimensionsFromEntity,
 } from './train-capacity.util';
 import {
@@ -288,31 +291,42 @@ export class TrainSchedulingService {
 
   async createContainerTrainSchedule(dto: CreateContainerTrainScheduleDto) {
     const route = await this.getActiveRoute(dto.routeId);
-    const locomotive = await this.selectOrValidateLocomotive(dto.locomotiveId, 0, 0);
+
+    const locomotiveIds = [...new Set(dto.locomotiveIds)];
+    if (locomotiveIds.length < 2) {
+      throw new BadRequestException('A train must be pulled by at least two locomotives');
+    }
 
     const createdScheduleId = await this.dataSource.transaction(async (manager) => {
-      const lockedLocomotive = await manager.getRepository(Locomotive).findOne({
-        where: { id: locomotive.id },
-        lock: { mode: 'pessimistic_write' },
-      });
-      if (!lockedLocomotive) {
-        throw new NotFoundException(`Locomotive ${locomotive.id} not found`);
-      }
-      if (lockedLocomotive.status !== 'AVAILABLE') {
-        throw new ConflictException(`Locomotive ${lockedLocomotive.code} is not available`);
+      // Lock and validate every locomotive: all must be AVAILABLE and at the origin yard.
+      const lockedLocomotives: Locomotive[] = [];
+      for (const locomotiveId of locomotiveIds) {
+        const locked = await manager.getRepository(Locomotive).findOne({
+          where: { id: locomotiveId },
+          lock: { mode: 'pessimistic_write' },
+        });
+        if (!locked) {
+          throw new NotFoundException(`Locomotive ${locomotiveId} not found`);
+        }
+        if (locked.status !== 'AVAILABLE') {
+          throw new ConflictException(`Locomotive ${locked.code} is not available`);
+        }
+        if (locked.currentYardId !== route.originYardId) {
+          throw new ConflictException(
+            `Locomotive ${locked.code} is at yard ${locked.currentYardId} but schedule originates from ${route.originYardId}`,
+          );
+        }
+        lockedLocomotives.push(locked);
       }
 
       const direction = deriveScheduleDirection(
         route.originYard ?? { country: null },
         route.destinationYard ?? { country: null },
       );
-      if (lockedLocomotive.currentYardId !== route.originYardId) {
-        throw new ConflictException(
-          `Locomotive ${lockedLocomotive.code} is at yard ${lockedLocomotive.currentYardId} but schedule originates from ${route.originYardId}`,
-        );
-      }
 
-      const trainSet = await this.buildEmptyTrainSet(manager, lockedLocomotive);
+      const trainSet = await this.buildEmptyTrainSet(manager, lockedLocomotives);
+      // Effective capacity is capped by the weakest locomotive in the set.
+      const limitLoco = minLocomotiveLimits(lockedLocomotives) ?? undefined;
       const schedule = manager.getRepository(TrainSchedule).create({
         trainSetId: trainSet.id,
         routeId: route.id,
@@ -322,11 +336,14 @@ export class TrainSchedulingService {
         status: TrainScheduleStatusEnum.Draft,
         direction,
         maxWagons: (
-          await this.resolveTrainLimitConfig(dto, lockedLocomotive)
+          await this.resolveTrainLimitConfig(dto, limitLoco)
         ).maxWagonsPerTrain,
       });
       const saved = await manager.getRepository(TrainSchedule).save(schedule);
-      await manager.getRepository(Locomotive).update(lockedLocomotive.id, { status: 'ASSIGNED' });
+      await manager.getRepository(Locomotive).update(
+        { id: In(lockedLocomotives.map((l) => l.id)) },
+        { status: 'ASSIGNED' },
+      );
       return saved.id;
     });
 
@@ -375,8 +392,9 @@ export class TrainSchedulingService {
       maxWagonsPerTrain: dto.maxWagonsPerTrain,
     };
 
-    const locomotive = schedule.trainSet.locomotive;
-    const limits = await this.resolveTrainLimitConfig(previewDto, locomotive ?? undefined);
+    const setLocomotives = this.locomotivesOfTrainSet(schedule.trainSet);
+    const limitLoco = minLocomotiveLimits(setLocomotives) ?? undefined;
+    const limits = await this.resolveTrainLimitConfig(previewDto, limitLoco);
     const validation = await this.validateBookingsForScheduling(
       previewDto,
       freightType ?? null,
@@ -408,17 +426,17 @@ export class TrainSchedulingService {
     const totalWeightTons = validation.summary.totalWeightTons;
     const totalLengthMeters = validation.summary.totalLengthMeters;
 
-    if (!locomotive) {
-      throw new BadRequestException('Schedule train set has no locomotive');
+    if (!limitLoco) {
+      throw new BadRequestException('Schedule train set has no locomotives');
     }
-    if (Number(locomotive.maxPullWeightTons) < totalWeightTons) {
+    if (limitLoco.maxPullWeightTons < totalWeightTons) {
       throw new BadRequestException(
-        `Locomotive ${locomotive.code} cannot pull ${totalWeightTons}T`,
+        `Train set locomotives cannot pull ${totalWeightTons}T`,
       );
     }
-    if (Number(locomotive.maxTrainLengthMeters) < totalLengthMeters) {
+    if (limitLoco.maxTrainLengthMeters < totalLengthMeters) {
       throw new BadRequestException(
-        `Locomotive ${locomotive.code} cannot support ${totalLengthMeters}m`,
+        `Train set locomotives cannot support ${totalLengthMeters}m`,
       );
     }
 
@@ -681,10 +699,12 @@ export class TrainSchedulingService {
 
     const now = new Date();
     await this.dataSource.transaction(async (manager) => {
+      const trainNumber = await this.assignTrainNumber(manager, schedule);
+
       await this.trainSchedulesRepository.updateStatus(
         scheduleId,
         TrainScheduleStatusEnum.Dispatched,
-        { actualDepartureAt: now },
+        { actualDepartureAt: now, trainNumber },
         manager,
       );
       if (schedule.trainSetId) {
@@ -718,6 +738,60 @@ export class TrainSchedulingService {
     return this.getTrainScheduleById(scheduleId);
   }
 
+  /**
+   * Assign a fixed train number on dispatch. The number is drawn from the pool
+   * for the train's dominant cargo type (container vs bulk) and trade direction
+   * (export = odd, import = even). Numbers recycle once a train ARRIVES, so the
+   * "used" set is every still-DISPATCHED schedule's number. Locked FOR UPDATE so
+   * concurrent dispatches can't grab the same number. Throws when the pool is
+   * exhausted. Idempotent: returns the existing number if already assigned.
+   */
+  private async assignTrainNumber(
+    manager: EntityManager,
+    schedule: TrainSchedule,
+  ): Promise<string> {
+    if (schedule.trainNumber) return schedule.trainNumber;
+
+    // Count container vs bulk wagons from the planned allocations.
+    let containerWagons = 0;
+    let bulkWagons = 0;
+    for (const wagon of schedule.trainSet?.wagons ?? []) {
+      const isBulk = (wagon.allocations ?? []).some((a) => a.loadType === 'BULK');
+      if (isBulk) bulkWagons += 1;
+      else containerWagons += 1;
+    }
+
+    const direction =
+      (schedule.direction as 'IMPORT' | 'EXPORT' | 'DOMESTIC' | null) ??
+      (schedule.originStation && schedule.destinationStation
+        ? deriveScheduleDirection(schedule.originStation, schedule.destinationStation)
+        : null);
+
+    const pool = pickTrainNumberPool(containerWagons, bulkWagons, direction);
+
+    // Lock the set of currently-active numbered schedules so two concurrent
+    // dispatches serialize and can't both claim the same lowest-free number.
+    const activeNumbered = await manager
+      .getRepository(TrainSchedule)
+      .createQueryBuilder('schedule')
+      .setLock('pessimistic_write')
+      .where('schedule.status = :status', { status: TrainScheduleStatusEnum.Dispatched })
+      .andWhere('schedule.train_number IS NOT NULL')
+      .getMany();
+
+    const usedNumbers = activeNumbered
+      .map((s) => s.trainNumber)
+      .filter((n): n is string => Boolean(n));
+
+    const number = pickLowestFreeNumber(pool.numbers, usedNumbers);
+    if (!number) {
+      throw new ConflictException(
+        `No free ${pool.cargo.toLowerCase()} ${pool.direction.toLowerCase()} train number available; a train must arrive to free one`,
+      );
+    }
+    return number;
+  }
+
   /** Open or close a schedule's booking window (staff override). */
   async setBookingWindow(scheduleId: string, status: 'OPEN' | 'CLOSED'): Promise<void> {
     await this.dataSource
@@ -931,16 +1005,12 @@ export class TrainSchedulingService {
         });
       }
 
-      if (schedule.trainSet?.locomotiveId) {
-        const loco = await manager
-          .getRepository(Locomotive)
-          .findOne({ where: { id: schedule.trainSet.locomotiveId } });
-        if (loco) {
-          await manager.getRepository(Locomotive).update(loco.id, {
-            status: 'AVAILABLE',
-            currentYardId: schedule.destinationStationId,
-          });
-        }
+      const arrivingLocoIds = this.locomotivesOfTrainSet(schedule.trainSet).map((l) => l.id);
+      if (arrivingLocoIds.length) {
+        await manager.getRepository(Locomotive).update(
+          { id: In(arrivingLocoIds) },
+          { status: 'AVAILABLE', currentYardId: schedule.destinationStationId },
+        );
       }
 
       for (const slot of schedule.trainSet?.wagons ?? []) {
@@ -982,7 +1052,7 @@ export class TrainSchedulingService {
   async getContainerTrainSchedules() {
     const schedules = await this.trainSchedulesRepository.findAll({
       relations: {
-        trainSet: { locomotive: true },
+        trainSet: { locomotive: true, locomotives: { locomotive: true } },
         route: true,
         originStation: true,
         destinationStation: true,
@@ -1013,10 +1083,11 @@ export class TrainSchedulingService {
       if (schedule.trainSetId) {
         await manager.getRepository(TrainSet).update(schedule.trainSetId, { status: 'CANCELLED' });
       }
-      if (schedule.trainSet?.locomotiveId) {
-        await manager.getRepository(Locomotive).update(schedule.trainSet.locomotiveId, {
-          status: 'AVAILABLE',
-        });
+      const cancelledLocoIds = this.locomotivesOfTrainSet(schedule.trainSet).map((l) => l.id);
+      if (cancelledLocoIds.length) {
+        await manager
+          .getRepository(Locomotive)
+          .update({ id: In(cancelledLocoIds) }, { status: 'AVAILABLE' });
       }
       for (const wagon of schedule.trainSet?.wagons ?? []) {
         if (wagon.physicalWagonId) {
@@ -1251,24 +1322,29 @@ export class TrainSchedulingService {
       }
     }
 
-    let assignedLocomotive: Locomotive | null = null;
+    let assignedLocomotives: Locomotive[] = [];
     if (targetScheduleId) {
       const targetSchedule =
         await this.trainSchedulesRepository.findByIdWithFullGraph(targetScheduleId);
-      assignedLocomotive = targetSchedule?.trainSet?.locomotive ?? null;
+      assignedLocomotives = this.locomotivesOfTrainSet(targetSchedule?.trainSet);
     }
 
-    if (assignedLocomotive) {
-      if (assignedLocomotive.currentYardId !== originYardId) {
+    if (assignedLocomotives.length) {
+      // Every locomotive of the set must sit at the origin yard, and the weakest
+      // one must still be able to pull the train (min limits across the set).
+      const offYard = assignedLocomotives.find((l) => l.currentYardId !== originYardId);
+      const setLimits = minLocomotiveLimits(assignedLocomotives);
+      if (offYard) {
         violations.push(
-          `Locomotive ${assignedLocomotive.code} is not at the schedule origin yard`,
+          `Locomotive ${offYard.code} is not at the schedule origin yard`,
         );
       } else if (
-        Number(assignedLocomotive.maxPullWeightTons) < totalWeightTons ||
-        Number(assignedLocomotive.maxTrainLengthMeters) < totalLengthMeters
+        setLimits &&
+        (setLimits.maxPullWeightTons < totalWeightTons ||
+          setLimits.maxTrainLengthMeters < totalLengthMeters)
       ) {
         violations.push(
-          'Assigned locomotive cannot support the total train weight and length',
+          'Assigned locomotives cannot support the total train weight and length',
         );
       }
     } else {
@@ -1818,6 +1894,22 @@ export class TrainSchedulingService {
     }
   }
 
+  /**
+   * All locomotives attached to a loaded train set. Prefers the `locomotives`
+   * link rows; falls back to the legacy single `locomotive` for train sets
+   * created before multi-loco support.
+   */
+  private locomotivesOfTrainSet(
+    trainSet: TrainSet | null | undefined,
+  ): Locomotive[] {
+    if (!trainSet) return [];
+    const linked = (trainSet.locomotives ?? [])
+      .map((link) => link.locomotive)
+      .filter((loco): loco is Locomotive => Boolean(loco));
+    if (linked.length) return linked;
+    return trainSet.locomotive ? [trainSet.locomotive] : [];
+  }
+
   async selectOrValidateLocomotive(
     locomotiveId: string,
     totalWeightTons: number,
@@ -1841,15 +1933,28 @@ export class TrainSchedulingService {
     return locomotive;
   }
 
-  private async buildEmptyTrainSet(manager: EntityManager, locomotive: Locomotive) {
+  private async buildEmptyTrainSet(manager: EntityManager, locomotives: Locomotive[]) {
+    const [primary] = locomotives;
     const trainSet = manager.getRepository(TrainSet).create({
-      locomotiveId: locomotive.id,
+      // `locomotiveId` retained as the primary locomotive for single-loco read paths.
+      locomotiveId: primary.id,
       totalWeightTons: 0,
       totalLengthMeters: 0,
       wagonCount: 0,
       status: 'DRAFT',
     });
-    return manager.getRepository(TrainSet).save(trainSet);
+    const saved = await manager.getRepository(TrainSet).save(trainSet);
+
+    const links = locomotives.map((loco, index) =>
+      manager.getRepository(TrainSetLocomotive).create({
+        trainSetId: saved.id,
+        locomotiveId: loco.id,
+        sequenceNo: index,
+      }),
+    );
+    await manager.getRepository(TrainSetLocomotive).save(links);
+
+    return saved;
   }
 
   private async getActiveRoute(routeId: string) {
@@ -1915,6 +2020,12 @@ export class TrainSchedulingService {
             currentYardId: schedule.trainSet.locomotive.currentYardId ?? null,
           }
         : null,
+      locomotives: this.locomotivesOfTrainSet(schedule.trainSet).map((loco) => ({
+        id: loco.id,
+        code: loco.code,
+        name: loco.name ?? null,
+        currentYardId: loco.currentYardId ?? null,
+      })),
       wagonCount: schedule.trainSet?.wagonCount ?? 0,
       totalWeightTons: roundTons(Number(schedule.trainSet?.totalWeightTons ?? 0)),
       totalLengthMeters: roundTons(Number(schedule.trainSet?.totalLengthMeters ?? 0)),
@@ -1952,7 +2063,7 @@ export class TrainSchedulingService {
         bookingWindowStatus: 'OPEN',
       },
       relations: {
-        trainSet: { locomotive: true },
+        trainSet: { locomotive: true, locomotives: { locomotive: true } },
         route: { milestones: true },
         originStation: true,
         destinationStation: true,
@@ -2099,6 +2210,15 @@ export class TrainSchedulingService {
                   ),
                 }
               : null,
+            locomotives: this.locomotivesOfTrainSet(schedule.trainSet).map((loco) => ({
+              id: loco.id,
+              code: loco.code,
+              name: loco.name ?? null,
+              status: loco.status,
+              currentYardId: loco.currentYardId ?? null,
+              maxPullWeightTons: roundTons(Number(loco.maxPullWeightTons)),
+              maxTrainLengthMeters: roundTons(Number(loco.maxTrainLengthMeters)),
+            })),
             wagons: [...(schedule.trainSet.wagons ?? [])]
               .sort((a, b) => a.sequenceNo - b.sequenceNo)
               .map((wagon) => ({