A small mounted device at the end of the train performs some important functions.
The device has a high visibility marker, a bright light to show other train crews where the end of the train is.
It measures the brake pipe pressure at the end of the train and transmits the information to the locomotive cabin, where the crew can see it. It also has a motion detector to tell the crew whether the end of the train is moving. A locomotive starting to pull a long train can move a good distance before the last car moves, so the motion detector lets the crew know the progress of the entire train.
The end-of-train device can actively assist with emergency braking. Pressure is dropped from the brake line at both ends of the train to activate the brakes, resulting in much faster deceleration than if the brakes were applied only at the front.