Electro-diesel locomotive

An electro-diesel locomotive (also referred to as a dual-mode or bi-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.

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 (using a diesel locomotive where electrified lines are available) 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 prime mover intended only for low-speed or short-distance 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 that used tramways at the ports of Southampton and Weymouth. For economy, the diesel engine and its generator are considerably smaller than the electric capacity. The Southern types were of 1,600 hp or 'Type 3' rating as electrics, but only 600 hp as diesels. Later classes had as much as 2,500 hp on electric power, but still the same diesel engines. Despite this large difference, their comparable tractive efforts were much closer (around three-quarters as diesels) and so they could start and work equally heavy trains as diesels, but not to the same speeds.

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, GE Genesis P32AC-DM, EMD DM30AC). The primary function for these models is to provide a "one-seat ride" (a rail trip that doesn't require a transfer to a different train) between the electrified and non-electrified sections of a rail system or to allow trains to run through tunnels or other segments of track where diesel locomotives are generally prohibited due to their production of exhaust; such locomotives are used for certain trains servicing the New York City terminals of Grand Central Terminal and Penn Station, as the various rail tunnels into Manhattan have exhaust restrictions. Once out of the tunnels, the engines are started and operation is as a normal diesel locomotive.

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