Just youtube “MU set-up” for locomotives. MU = multi-unit.
We had a few trial runs with ECP coal trains years ago. In this air brake system, there was a communication line stretching the entire length of the train. When drawing the brakes, each car would exhaust and apply simultaneously instead of having to wait until the entire trainline exhausted from the front to the rear. IIRC, passenger trains have a similar system in order to apply the brakes much faster on the train. This is opposed to a standard freight train system that would take much longer to achieve a full service brake pipe reduction.