What should be the first action taken when smoke is observed in the tunnel while operating a PA5 train?

Stopping the train, calling the trainmaster, and shutting off the Fresh Air Dampers is the most appropriate first action when smoke is observed in the tunnel while operating a PA5 train. This response prioritizes safety by ensuring that the situation is assessed and controlled correctly.

Stopping the train allows for immediate assessment of the situation, which is crucial in emergencies involving smoke. Maintaining communication through the trainmaster enables coordination of a safe response and ensures that appropriate emergency protocols are activated. Shutting off the Fresh Air Dampers helps prevent smoke from entering the train and potentially harming passengers, which can be critical in minimizing exposure to harmful substances.

Other options may seem appealing, but they do not prioritize safety or effective communication. For instance, increasing speed to clear the area might expose passengers to unnecessary risk and could spread the smoke further, while activating the emergency communication system comes after ensuring that the train is stopped and passengers are safe. Conducting an immediate passenger evacuation is a critical step but should follow the assessment and control of the situation to ensure it is done safely and effectively. Therefore, the course of action in option C aligns with emergency response protocols focused on safety and effective management of smoke-related incidents.