A device attached to the last coupler on the rear of a train consisting of a flashing light and a radio-controlled dump valve for activating the emergency brakes remotely, as well as telemetry circuitry for determining train length and a trainline air pressure sensor. When linked to a Head of Train Device (HTD), the HTD will display train length and the air pressure at the rear of the train to the Locomotive Engineer.
Modern EOT devices are typically powered by air turbines connected to the trainline via the end air hose on the last car.
If an emergency air brake application is desired, the engineer can flip a switch on the HTD which will instruct the EOT to open the dump valve, thus triggering the emergency application. By also venting the brakes from the head end, the time to apply the emergency brakes can be reduced (improving stopping distance) while affording the engineer some degree of control over the slack in their train.
US Federal Regulations require trains to have a flashing light at the rear of the train if operated at night, but during the day a simple flag can be used with some restrictions.
There are devices for the rear end that simply consist of a flashing light, but without the ability for an engineer to remotely initiate an emergency from the head end, freight trains are restricted to 30 mph by federal regulation (49 CFR § 232.407).