An Electro-diesel locomotive (also referred to as dual-mode locomotive) is powered either from an electricity supply (like an electric locomotive) or by using the onboard diesel engine (like a diesel-electric locomotive). For the most part, these locomotives are built to serve regional, niche markets with a very specific purpose.
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Electro-diesel locomotives and electro-diesel multiple units are used to provide continuous journeys along routes that are only partly electrified without a change of locomotive, extensive running of diesel under the wires or where diesel engines are banned. They may be designed or adapted mainly for electric use, mainly for diesel use or to work well as either electric or diesel.
This is effectively an electric locomotive with a relatively small auxiliary diesel engine intended only for low-speed or short-term operation (e.g. British Rail Class 73). Some of these, such as the British Rail Class 74, were converted from electric locomotives. The Southern Region of British Railways used these locomotives to cross non-electrified gaps and to haul boat trains which used tramways at the ports of Southampton and Weymouth.
This is effectively a diesel locomotive with auxiliary electric motors (or connections to the existing traction motors), usually operating from 750 V DC third rail where non-electric traction is banned (e.g. EMD FL9). The primary function for these models is to provide a one-seat ride between diesel territory and the New York City terminals of Grand Central Terminal and Penn Station, where diesel exhaust is prohibited due to the confined tunnel space. Once out of the tunnels, the third-rail shoes are retracted and operation is as a normal diesel locomotive.
With modern electronics, it is much easier to construct (or adapt) an electro-diesel locomotive or multiple-unit which is equally at home running at high speeds both under the wires and under diesel power (e.g. SNCF Class B 82500). These will normally operate under pure electric traction where possible, and use the diesel engines to extend the journeys along non-electrified sections which would not be cost effective to electrify. They may also be used on long cross-country routes to take advantage of shorter sections of electrified main lines.
Bombardier has built dual-mode variants of its AGC series for the French operator SNCF; the electricity is collected by means of a pantograph.
In Russia, a number of electro-diesels were built which had both pantographs and diesel motors; these locomotives were actually multiple-units, with distinct electric, diesel, and motor sections. These included:
An experimental electro-diesel locomotive, DEL120, was built by London Underground in 1940 but was not a success. Only two other types have been built. In both cases, their electricity source was a 750 V DC third rail.
The Hitachi Super Express trains proposed for the electrification of the Great Western Main Line have a variant able to operate in bi-mode as an electro-diesel trainset. This would enable intercity services from Paddington Station to continue beyond Cardiff Central railway station, currently the western limit of the proposed electrification scheme.
Bombardier has plans,[4] apparently known as Operation Thor or Project Thor, to convert 500 existing diesel multiple units (DMUs) into electro-diesel multiple units (EDMUs).[5] Whilst exact details are unclear, one source specifies that these would involve their Voyager family DEMUs, and entail the construction of additional intermediate cars with a pantograph and a transformer which would feed power into the traction motors of the existing diesel-electric cars when running off overhead lines.[6]
Several primarily diesel locomotive types and a multiple-unit have been built to operate off a 750 VDC third rail into the New York City terminals of Grand Central Terminal and Penn Station:
Additionally, a full dual-mode 200/160 km/h locomotive for use with catenary is now under test[7].
The South African Class 38-000 is a 3kV DC electro-diesel locomotive designed by Consortium under the leadership of Siemens and built by Union Carriage and Wagon (UCW) in Nigel, Gauteng, South Africa. Between November 1992 and 1993 fifty of these locomotives were placed in service by Spoornet, formerly the South African Railways (SAR) and later renamed Transnet Freight Rail (TFR). The diesel engine enables the locomotive to shunt on unelectrified sidings.
A specialized type of electro-diesel locomotive is the hybrid locomotive. Here, the electricity comes from a battery (charged by the diesel engine) rather than an external supply. An example is the Green Goat switcher[10].
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