Final answer:
When following a regular passenger train, a train operator taking a lay-up train to the yard must be careful to maintain a safe distance and follow signals to prevent accidents.
Step-by-step explanation:
When a train operator is taking a lay-up train to the yard and following a regular passenger train enroute, they must be especially careful to maintain a safe distance and avoid any collisions or accidents. This is because the regular passenger train may need to make sudden stops or unexpected movements. The train operator should also follow the signals and instructions from the regular passenger train to ensure a smooth and safe journey.