Which valve at each car senses the increase in pressure when the brakes are released?

The control valve is responsible for sensing the increase in pressure when the brakes are released. When a train's braking system is engaged, the control valve modulates the flow of air to the brake cylinders. Upon the release of the brakes, this valve detects the change in air pressure, which confirms that the brake system is returning to its normal state. The proper functioning of the control valve is crucial as it impacts the overall efficiency and responsiveness of the train's braking system.

In contrast, the automatic brake valve manages the application and release of the brakes from the engineer's position but does not perform pressure sensing at each car. The emergency valve is designed to facilitate the rapid application of the brakes in an emergency situation but does not monitor pressure changes in a regular braking context. The service release valve aids in releasing the brakes but focuses primarily on helping the system reset rather than sensing pressure changes. This distinction highlights the control valve's specific role in monitoring pressure changes effectively during brake operations.