At center is a narrow box affixed to the freight car's coupler. It is an end-of-train device (EOTD), also known as a flashing rear-end device (FRED). It performs a function formerly done by humans riding in a caboose: it informs the engineer by telemetry of the air pressure in the brake line at the end of the train.

Knowing the air pressure at the end of a train is akin to knowing if a car’s parking brake is on: it makes no sense to try to go forward unless all brakes are released. When the engineer releases the train's air brakes (by pumping air into the brake line from a pump in a locomotive), the effect is not instantaneous throughout a long train. The rear end might feel the effect minutes after the engineer's action. Thus the engineer must wait until the EOTD informs him or her that the air pressure at the end of the train is sufficient to have released all brakes on the train.

During hours of darkness, an EOTD also flashes a red light. It is powered by a battery.