thumb|An ETD on a [[containerization|container train in 2005]]
The end of train device (ETD), sometimes referred to as an EOT, flashing rear-end device (FRED) or sense and braking unit (SBU) is an electronic device mounted on the end of freight trains in replacement of a caboose. They are divided into three categories: "dumb" units, which only provide a visible indication of the rear of the train with a flashing red taillight; "average intelligence" units with a brake pipe pressure gauge; and "smart" units, which send back data to the crew in the locomotive via radio-based telemetry.
Tail lamps
thumb|A tail lamp on the last carriage of a train, showing that the train is complete
The earliest known method of showing the position of and completeness of a train was by the Stockton and Darlington Railway in the 1830s, where a burning brazier was attached to the rear of trains.
Under the absolute block system, each train must carry a tail lamp with a red lens on the rear. The purpose of the tail lamp is to show the signaller that the train has not separated in the previous block section, and is the only guarantee that the train is complete. Trains not fitted with a continuous brake were also required to carry side lamps on the rearmost vehicle; these lamps pointed forwards and showed a white light. Side lamps showed the driver of the train that their train was complete and had not separated.
End of train devices
Design and use
A "dumb" ETD can be as simple as a red flag attached to the coupler on the last car of the train, whereas "smart" devices monitor functions such as brake line pressure and accidental separation of the train using a motion sensor, functions that were previously monitored by a crew in the caboose. The ETD transmits data via a telemetry link to the Head-of-Train Device (HTD) in the locomotive, known colloquially among railroaders as a "Wilma," after cartoon character Wilma Flintstone. In Canada, this device is known as a sense and braking unit (SBU).
A typical HTD contains several lights indicating telemetry status and rear end movement, along with a digital readout of the brake line pressure from the ETD. It also contains a toggle switch used to initiate an emergency brake application from the rear end. In modern locomotives, the HTD is built into the locomotive's computer system, and the information is displayed on the engineer's computer screen.
Railroads have strict government-approved air brake testing procedures for various circumstances when assembling trains or switching out cars en route. After a cut is made between cars in a train and the train is rejoined, in addition to other tests, the crew must verify that the brakes apply and release on the rear car (to ensure that all of the brake hoses are connected and the angle cocks, or valves, are opened). In most cases, the engineer is able to use information from the ETD to verify that the air pressure reduces and increases at the rear of the train accordingly, indicating proper brake pipe continuity. This device is said to constitute a fail-safe condition.
The DPS ETD reduced labor costs, as well as the costs of the purchase and upkeep of cabooses. The Brotherhood of Conductors and Brotherhood of Railroad Brakemen were also greatly affected by ETD, as this electronic unit replaced two crewmen per train. The widespread use of ETDs has made the caboose nearly obsolete. Some roads still use cabooses where the train must be backed up, on short local runs,