Final answer:
Train operators must stop moving and become stationary when approaching standing cars in a barn to ensure safety.
Step-by-step explanation:
The safety stops that a train operator must make when approaching standing cars in a barn are to stop moving and become stationary until it passes by. This is to ensure the safety of both the train operator and any passengers or workers near the standing cars. By stopping and waiting for the cars to pass, the train operator can prevent any accidents or collisions.