What action must be taken if a train overruns a block signal that requires a stop?

When a train overruns a block signal that indicates a stop, the situation poses a significant safety risk, and immediate communication is essential. The correct response is to warn other trains at once by radio. This action helps ensure the safety of all train operations in the area by informing other crews about the situation. It assists in preventing potential collisions or further incidents by providing critical information to anyone who might be entering the affected block.

In this scenario, the necessity of quick communication cannot be overstated. The surrounding trains and crews need to be aware of the overrunning to adjust their speed and operations accordingly, maintaining safety on the tracks.

Other actions like continuing to the next block signal or reporting the incident to the engineer are not appropriate responses in this critical moment, as they do not prioritize the immediate safety of other train operations. Contacting the maintenance team, while it may be necessary as a follow-up procedure, does not address the immediate danger presented by the train overrunning the signal. Thus, warning other trains is the most crucial and immediate response required in such situations.