What happens when the emergency cord is pulled on a subway car?

When the emergency cord is pulled on a subway car, the primary function it serves is to alert the conductor about a potential emergency situation, making the choice of activating the brake system the correct answer for this scenario.

Pulling the emergency cord triggers an alert signal that is sent to the train's control systems, which ultimately activates the braking system to bring the train to a halt. This action ensures the safety of passengers and allows for a swift response to any emergencies. While an air valve might be part of the braking system's operation, the immediate and critical consequence of pulling the emergency cord is the activation of brakes to stop the train safely.

This reaction is vital for maintaining safety protocols in the subway system, prioritizing the well-being of passengers and staff aboard the train.