Head-end power
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.
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.
The resulting separate systems of lighting power, steam heat, and engine-driven air conditioning, increased the maintenance workload as well as parts proliferation.
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.
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, providing connections to the cars.
Some early diesel streamliners took advantage of their fixed-consist construction to employ electrically powered lighting, air conditioning, and heating.
For example, the Nebraska Zephyr trainset has three diesel generator sets in the first car to power onboard equipment.
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.
While many commuter fleets were quickly converted to HEP, long-distance trains continued to operate with steam heat and battery-powered electrical systems.
Amtrak subsequently converted a portion of the steam-heated fleet to all-electric operation using HEP, and retired the remaining unconverted cars by the mid-1980s.
[7] Some head-end power cars started out as other forms of rolling stock that have been rebuilt with diesel generators and fuel tanks to supply HEP to the passenger equipment.
[8][9] Although diesel-powered cars are more common, electric ones also exist and are used to provide power to trains when hauled by locos without HEP, or when not attached to a locomotive.
Genset-supplied HEP is usually through an auxiliary diesel unit that is independent from the main propulsion (prime mover) engine.
[10] More recently, locomotives have adopted the use of a static inverter, powered from the traction generator, which allows the prime mover to have a larger RPM range.
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.
[11] HEP power supplies the lighting, HVAC, dining car, kitchen, and battery charging loads.
[12] Because of train lengths and the high power requirements in North America, HEP is supplied as three-phase AC at 480 V (standard in the US), 575 V, or 600 V. Transformers are fitted in each car for reduction to lower voltages.
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.
The Enterprise Dublin-Belfast train sets initially used HEP from GM 201 diesel-electric locomotives, but due to reliability issues and excessive wear on the locomotives systems, generator vans (sourced from retired Irish Rail MK3 sets and adapted for push-pull use) were added.
Older European cars used high voltage only for heating, while light, fans and other low-current supply (e.g. shaver sockets in bathrooms) power was provided by axle-driven generator.
Although considered inefficient and obsolete, mainly because the generator car 'wastes' traction power, staff, and fuel (if running on electrified lines), new cars using AC HEP are still in production, along with new generator cars/sets, mostly for use in areas without electrification, considering that the vast majority of China Railways' engines that are capable of supplying HEP are electric locomotives.
Small number of special generator cars (QZ-KD25T) designated for use on the high-altitude Qinghai–Tibet Railway also supply power at 600 V DC.
With new DC-equipped engines and cars entering service rapidly, as well as ageing and retirement of older equipments using AC, DC HEP has become the more prominent form of power supply of China Railways.
Where a passenger train must be hauled by a locomotive with no HEP supply (or an incompatible HEP supply) a separate generator van may be used [13] such as on the Amtrak Cascades train or Iarnród Éireann's CAF Mark 4 Driving Van Trailer (with twin MAN 2846 LE 202 (320 kW) / Letag (330 kVA) engine / generator sets, assembled by GESAN).
