If any part of a train passes a STOP signal without authority, what must a crew member do?

If any part of a train passes a STOP signal without authority, the appropriate action is to warn any other movement that could be approaching. This action is crucial for maintaining safety on the railway. When a train disregards a STOP signal, it may create a hazardous situation for other trains in the vicinity, especially if they are on the same track or are scheduled to approach that section. Therefore, it is vital for a crew member to alert others to prevent potential collisions or accidents. This warning can be communicated through various methods, including radio contact, signals, or alarms, ensuring that everyone in the area is aware of the situation and can take necessary precautions.

The other options do not align with proper safety protocols. Accelerating in such a scenario would only exacerbate the risk of an accident. Ignoring the signal is unacceptable as it undermines the very function of signals, which is to ensure safe train operations. Thus, the emphasis on warning others reflects the critical importance of communication and safety in railway operations.