Head end power

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Head end power (also known as hotel power, or Electric train supply (ETS), in British railway terminology) is a method of providing electricity to the carriages of a train, usually the passenger carriages of a long distance 'hotel' train. A generator or alternator in a power car usually provides this power, which then runs through various cabling and connectors along most of the train, but often excluding the locomotives and baggage cars. The power provided from HEP is used for lighting, heating, ventilation and air conditioning (HVAC), the dining car kitchen, on-board entertainment systems, and various other uses, which may include power outlets for shavers and laptops and hand dryers in toilets.

The name "Head End Power" comes from the fact that many times it is the locomotive that provides the power, and popular railway jargon in the USA calls the front of the train the "head end." However, power cars may be placed in any location on the train, not necessarily the front of the train.

The generator/alternator may be powered from the power car's prime mover or it may be powered by a separate engine used solely to power the generator/alternator. This separate generator can also be in a separate car from the locomotive. The former arrangement is less expensive but requires that the prime mover turn at a constant RPM whereas the latter arrangement allows the prime mover to change speeds to meet the changing traction load or even to be shut down. Occasionally, this arrangement even changes over the life of a given locomotive; see EMD F40PH for one example.

Because of the length of trains and high power requirements, it is usually advantageous to supply the HEP as polyphase AC at high voltage, with transformers in each car for reduction to usable voltages. Voltages in 480, 575, and 600 volts are common. The HEP may also charge batteries on the cars, to temporarily supply power when HEP is not available or when cars are un-coupled.

Engines for separate-engine powered HEP Generator may vary, but in the US, they are mainly the Caterpillar 3412 V12 and the Cummins K-Series Inline 6, but smaller under-car engines for powering short trains are also manufactured. Stadco is one popular brand of under-car generator in the US.