If a portion of a train must be temporarily left on the main track, what must be done to the detached portion?

The correct action when a portion of a train must be temporarily left on the main track is to set a sufficient number of hand brakes. This is essential for ensuring the safety and security of the detached portion. Setting hand brakes is a critical practice designed to prevent unintended movement of the train segment, especially on grades where gravity could cause it to roll. The number of hand brakes needed depends on the weight of the detached portion and the gradient of the track, ensuring that the train remains securely positioned until it can be safely moved or reattached.

In the context of railway operations, safety is paramount, so securing any detached portion with hand brakes provides immediate risk mitigation. Other options, while related to the situation, do not directly address the immediate need to secure the train. For example, removing cabooses or notifying the dispatcher are steps that may follow after securing the train, but they do not ensure the safety of the piece left on the track. Furthermore, moving the detached portion off the main track is a necessary objective, but it cannot happen until the train is securely stopped to prevent accidents during the process.