What procedure does a watchman use to alert others of an approaching train?

The correct answer is the train approach warning. This procedure is specifically designed for watchmen to effectively communicate the imminent arrival of an approaching train to those working in proximity to the tracks. The train approach warning is a critical safety measure that helps to ensure that all workers are alerted and can take necessary precautions to either clear the area or prepare for the train's arrival. This system relies on standardized signals and communication protocols that are well understood among those working on or near the railroad.

While other options such as signal alert, emergency broadcast, and radio communication may also play roles in railway safety, they do not specifically pertain to the direct method employed by a watchman for alerting workers about a train approaching. Thus, the train approach warning is recognized as the most appropriate and relevant procedure in this context, highlighting the importance of clear and immediate communication in ensuring the safety of roadway workers.