What action must be taken when an engineer operates a train with a failed on-board PTC apparatus?

When an engineer operates a train with a failed on-board Positive Train Control (PTC) apparatus, the appropriate action is to operate the train at restrictive speed. This protocol is in place to ensure the safety of the train operation when PTC, which is designed to prevent collisions and enforce speed limits, is not functioning.

Operating at restrictive speed allows the engineer to maintain a higher level of control over the train's movements, minimizing risks associated with potential signal violations or overspeed conditions that PTC would typically manage. It acts as a safeguard to ensure that the train still adheres to the safe operating procedures even without the additional safety measures provided by the PTC system.

In the context of railroad operations, maintaining safety and adhering to protocols during equipment failures is a critical aspect of ensuring safe train operation, which underscores why this choice is viewed as the correct action in such situations.