When an emergency brake is applied, which system is immediately affected?

When an emergency brake is applied, the friction brake is immediately affected because this system is directly responsible for rapidly decelerating the train. The emergency braking system is designed to engage the friction brakes, which utilize pads or shoes that press against the wheels or brake discs to create the necessary stopping force. This action is crucial for ensuring passenger safety during emergency situations and is typically triggered by a significant event that requires the train to stop as quickly as possible.

In comparison, other options relate to systems that either support passenger operations or functions that are not directly linked to the immediate stopping of the train. Passenger services being halted or non-essential systems shutting down may occur after the initial brake engagement, but they are secondary consequences rather than the direct effect of applying the emergency brakes. Similarly, putting the train into autopilot mode does not apply, as the focus during an emergency situation is to stop the train rather than to maintain its operation in an automated state.