In rail transport, head-end power (HEP), also known as electric train supply (ETS) is the electrical power distribution system on a passenger train. The power source, usually a locomotive (or a generator car) at the front or 'head' of a train, provides the electricity used for heating, lighting, electrical and other 'hotel' needs. The maritime equivalent is hotel electric power. A successful attempt by the London, Brighton and South Coast Railway in October 1881 to light the passenger car between London and Brighton heralded the beginning of using electricity to light trains in the world.
The North British Railway in 1881 successfully generated electricity using a dynamo on the Brotherhood steam locomotive to provide electrical lighting in a train, a concept that was later called head-end power. High steam consumption led to abandonment of the system. Three trains were started in 1883 by London, Brighton and South Coast Railway with electricity generated on board using a dynamo driven from one of the axles. This charged a lead-acid battery in the guard's van, and the guard operated and maintained the equipment. The system successfully provided electric lighting in the train.
Throughout the remainder of the age of steam and into the early diesel era, passenger cars were heated by low pressure saturated steam supplied by the locomotive, with the electricity for car lighting and ventilation being derived from batteries charged by axle-driven generators on each car, or from engine-generator sets mounted under the carbody. Starting in the 1930s, air conditioning became available on railcars, with the energy to run them being provided by mechanical power take offs from the axle, small dedicated engines or propane.
Originally, trains hauled by a steam locomotive would be provided with a supply of steam from the locomotive for heating the carriages. When diesel locomotives and electric locomotives replaced steam, the steam heating was then supplied by a steam-heat boiler. This was oil-fired (in diesel locomotives) or heated by an electric element (in electric locomotives). Oil-fired steam-heat boilers were unreliable. They caused more locomotive failures on any class to which they were fitted than any other system or component of the locomotive, and this was a major incentive to adopt a more reliable method of carriage heating.
Later diesels and electric locomotives were equipped with Electric Train Heating (ETH) apparatus, which supplied electrical power to the carriages to run electric heating elements installed alongside the steam-heat apparatus, which was retained for use with older locomotives. Later carriage designs abolished the steam-heat apparatus, and made use of the ETH supply for heating, lighting, ventilation, air conditioning, fans, sockets and kitchen equipment in the train. In recognition of this ETH was eventually renamed Electric Train Supply (ETS).
The first advance over the old axle generator system was developed on the Boston and Maine Railroad, which had placed a number of steam locomotives and passenger cars into dedicated commuter service in Boston. Due to the low average speeds and frequent stops characteristic of a commuter operation, the axle generators' output was insufficient to keep the batteries charged, resulting in passenger complaints about lighting and ventilation failures. In response, the railroad installed higher capacity generators on the locomotives assigned to these trains, and provided electrical connections to the cars. The cars used steam from the locomotive for heating.
When diesel locomotives were introduced to passenger service, they were equipped with steam generators to provide steam for car heating. However, the use of axle generators and batteries persisted for many years. This started to change in the late 1950s, when the Chicago and North Western Railway removed the steam generators from their EMD F7 and E8 locomotives in commuter service and installed diesel generator sets (see Peninsula 400). This was a natural evolution, as their commuter trains were already receiving low-voltage, low-current power from the locomotives to assist axle generators in maintaining battery charge.
Following its formation in 1971, Amtrak's initial locomotive purchase was the Electro-Motive (EMD) SDP40F, an adaptation of the widely used SD40-2 3000 horsepower freight locomotive, fitted with a passenger style carbody and steam generating capability. The SDP40F permitted the use of modern motive power in conjunction with the old steam-heated passenger cars acquired from predecessor railroads, allowing Amtrak time to procure purpose-built cars and locomotives.
In many applications, the locomotive's prime mover provides both propulsion and head-end power. If the HEP generator is driven by the engine then it must run at a constant speed (RPM) to maintain the required 50 Hz or 60 Hz AC line frequency. An engineer will not have to keep the throttle in a higher run position, as the onboard electronics control the speed of the engine to maintain the set frequency.
When derived from the prime mover, HEP is generated at the expense of traction power. For example, the General Electric 3,200 hp (2.4 MW) P32 and 4,000 hp (3.0 MW) P40 locomotives are derated to 2,900 and 3,650 hp (2.16 and 2.72 MW), respectively, when supplying HEP. The Fairbanks-Morse P-12-42 was one of the first HEP equipped locomotives to have its prime mover configured to run at a constant speed, with traction generator output regulated solely by varying excitation voltage.
In the UK, ETS is supplied at 800 V to 1000 V AC/DC two pole (400 or 600 A), 1500 V AC two pole (800 A) or at 415 V 3 phase on the HST. On the former Southern Region, Mk I carriages were wired for a 750 V DC supply. This corresponds to line voltage on the Third Rail network. Class 73 Locomotives simply supply this line voltage direct to the ETS jumpers, whilst Class 33 Diesel Electric Locomotives have a separate engine driven Train Heating Generator which supplies 750 V DC to the train heating connections.