Head end power (HEP) or electric train supply (ETS) is a rail transport term for the electrical power distribution system on a passenger train. The power source, usually a locomotive at the front or “head” of a train or a generator car, generates all the electricity used for lightening, electrical and other "hotel" needs. The maritime equivalent is Hotel Electric Power (HEP).
UK
Originally, trains hauled by a steam locomotive would be provided with a supply of steam from the locomotive's boiler 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 appallingly 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.
At this time, lighting was powered by batteries which were charged by a dynamo underneath each carriage when the train was in motion, and buffet cars would use bottled gas for cooking and water heating.
On modern Diesel multiple unit trains, such as the Virgin Trains Voyager, the engine mounted below each vehicle provides power for that vehicle.
Electric Train Heat (ETH) and Electric Train Supply (ETS)
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 not only for heating, but also to power lighting, ventilation, air conditioning, fans, sockets and kitchen equipment in the train. In recognition of this ETH was eventually renamed Electric Train Supply (ETS).
Each coach has an index relating to the maximum consumption of electricity that that coach could use. The sum of all the indices must not exceed the index of the locomotive. One "ETH index unit" equals 5kW; a locomotive with an ETH index of 95 can supply 475kW of electrical power to the train.
USA
During the age of steam, cars were heated by low pressure saturated steam supplied by the locomotive. Electricity for car lighting and ventilation was derived from batteries charged by axle-driven generators on each car or from engine-generator sets mounted under the carbody.
The first advance over this 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. It was discovered that due to the low average speeds and frequent stops characteristic of commuter operation, axle generators did not produce enough output to keep the batteries adequately charged, resulting frequent passenger complaints about lighting and ventilation failures. In response, the railroad fitted higher capacity generators to the locomotives assigned to pull these trains and arranged electrical connections to transmit the generators' output back to the cars. The cars still depended on 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, during which time 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. This was a natural evolution, as their commuter trains were already receiving low voltage, low amperage power from the locomotives to assist axle generators in maintaining battery charge. In some cases, commuter cars were equipped with propane engine-powered air conditioning. The resulting separate systems of lighting power, steam heat, and engine-driven air conditioning increased the maintenance workload, as well as parts proliferation, thus leading to the full-scale adoption of HEP, where a single power source would handle all these functions.
While commuter fleets were quickly converted to HEP, long distance trains continued to operate with steam heat and battery-powered electrical systems. This gradually changed following the transfer of intercity passenger rail service to Amtrak, ultimately resulting in full adoption of HEP in the USA and the discontinuation of the old systems.
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 rolling stock acquired from private railroads, giving Amtrak time to procure purpose-built cars and locomotives.
In 1975, Amtrak started to take delivery of the all-electric Amfleet car, hauled by General Electric (GE) P30CH and, later, EMD F40PH locomotives, both unit types being equipped to furnish HEP. Following the introduction of the Amfleet fleet, the (also all-electric) Superliner was placed into operation for servicing long-distance western routes. Amtrak subsequently converted a portion of the steam heated fleet to all-electric operation using HEP and retired the remaining unconverted cars.
Engine
The HEP generator can be driven by either a separate engine mounted in the locomotive or generator car, or by the locomotive's prime mover.
Separate engines
Engine types vary, but in the US, they are mainly Caterpillar 3412 V12 and Cummins K-Series Inline 6 models. In the past, Detroit Diesel 8V-71 and 12V-71 engines were also used. Such engine/generator sets are generally installed in a compartment in the rear of the locomotive that is isolated from the main engine room, drawing fuel from the locomotive's fuel tanks.
Smaller under-car engine/generator sets for providing electricity on short trains are also manufactured, Stadco being one popular brand.
Locomotive prime mover
In many applications, the locomotive's prime mover provides both propulsion and head end power. In most cases, the prime mover must run at a constant speed (RPM) to maintain the required 50 Hz or 60 Hz AC line frequency. For example, an EMD locomotive operating in HEP mode will run the prime mover at a constant 900 RPM (which is full RPM), driving the generator at 1500 RPM (50 Hz) or 1800 RPM (60 Hz) through a gearbox. For noise reduction purposes, the locomotive's main (traction) generator can also be configured to supply HEP, usually at 600 or 720 RPM. However, this mode is only available when the locomotive is stationary.
The advent of power electronics has allowed the prime mover to operate over a larger speed range and still supply a constant HEP voltage and frequency by means of inverters.
When derived from the prime mover, HEP is generated at the expense of traction power. For example, the General Electric 3200 horsepower (2.4 MW) P32 and 4000 horsepower (3.0 MW) Genesis-Series P40 locomotives are derated to 2900 (2.2 MW) and 3650 horsepower (2.72 MW), respectively, when supplying HEP.
Electrical loading
HEP power supplies the lighting, HVAC, dining car kitchen and battery charging loads. Individual car electrical loading ranges from 20 kW for a typical car to more than 150 kW for a Dome car with kitchen and dining area, such as Princess Tours Ultra-Dome cars operating in Alaska.
Because of the lengths of trains and the high power requirements, HEP is supplied, in North America, as three-phase AC at 480-V (standard in the US and for Canada's VIA), 575-V (GO Transit, Toronto), or rarely 600-V. Transformers are fitted in each car for reduction to lower voltages.
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
Alternatives
Although most locomotive-hauled trains take power directly from the locomotive, there have been examples (mainly in continental Europe) where restaurant cars would take power directly from the overhead wires.
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