What should be done first if smoke is observed in the tunnel while operating a PA5 train?

Stopping the train is the critical first step when smoke is observed in the tunnel while operating a PA5 train. This action is essential for several reasons. First and foremost, stopping the train ensures the safety of passengers and crew by avoiding further travel into potentially hazardous conditions. It allows for immediate assessment of the situation and the implementation of emergency procedures.

By halting the train, the crew can quickly evaluate whether the smoke is indicative of a fire or another issue, and it provides an opportunity to manage the safety of everyone on board. Once the train is stopped, the crew can then proceed with activating alarms, notifying emergency services, and ensuring that passengers are briefed and potentially evacuated if necessary.

Prioritizing the cessation of movement helps to minimize the risk of injury and maintains control over the situation, which is paramount in any emergency scenario.