What must the engineer do if they lose the ability to use the main engine's horn?

If the engineer loses the ability to use the main engine's horn, the appropriate action is to utilize alternative signaling methods until repairs can be made. The horn serves as a critical safety feature for alerting others to the train's presence and intentions, particularly in areas where visibility may be limited or where people might be near the tracks.

By using alternative signaling methods, the engineer can continue to communicate essential signals to both personnel and the surrounding environment. This may involve utilizing hand signals, lights, or other means that are acceptable under safety regulations. The goal is to maintain safety and operational integrity while addressing the incapacity of the primary signaling mechanism until the train can be repaired or the situation resolved.

The other choices, such as stopping the train and waiting for repairs, communicating only through radio, or proceeding cautiously, could hinder operations or may not provide effective means of ensuring safety in the absence of the horn. Thus, employing alternative signaling methods is the most responsible and effective solution.