A locomotive or engine is a self-propelled, vehicular engine, powered by steam, a diesel, or electricity, for pulling or, sometimes, pushing a train or individual railroad cars. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, motor coach, railcar or power car; the use of these self-propelled vehicles is increasingly common for passenger trains, but rare for freight trains. Traditionally, locomotives pulled trains from the front. However, push-pull operation has become common, where the train may have a locomotive (or locomotives) at the front, at the rear, or at each end.

Wheel Designation

The Whyte classification system uses a combination of letters and numbers to describe the wheel arrangement of a steam locomotive. The first letter indicates the number of leading wheels, the number in the middle indicates the number of driving wheels, and the last letter indicates the number of trailing wheels. For example, a locomotive with 4 leading wheels, 6 driving wheels, and 2 trailing wheels would be classified as a 4-6-2.

The AAR wheel arrangement system is a method of classifying locomotive (or unit) wheel arrangements that was developed by the Association of American Railroads. Essentially a simplification of the European UIC classification, it is widely used in North America to describe diesel and electric locomotives (including third-rail electric locomotives). It is not used for steam locomotives, (except geared steam locomotives, which are instead classified by their model and their number of trucks), which use the Whyte notation instead.

Type of Engines

• Steam - A steam locomotive is a locomotive whose primary power source is a steam engine. The most common form of steam locomotive also contains a boiler to generate the steam used by the engine.

• Diesel - Diesel locomotives are powered by diesel engines. In the early days of diesel propulsion development, various transmission systems were employed with varying degrees of success, with electric transmission proving to be the most popular.

• Electric - An electric locomotive is a locomotive powered only by electricity. Electricity is supplied to moving trains with a (nearly) continuous conductor running along the track that usually takes one of three forms: an overhead line, suspended from poles or towers along the track or from structure or tunnel ceilings; a third rail mounted at track level; or an onboard battery

• Fireless - A fireless locomotive is a type of locomotive which uses reciprocating engines powered from a reservoir of compressed air or steam, which is filled at intervals from an external source.

• Atomic-electric - In the early 1950s, Lyle Borst of the University of Utah was given funding by various US railroad line and manufacturers to study the feasibility of an electric-drive locomotive, in which an onboard atomic reactor produced the steam to generate the electricity.

• Fuel cell-electric - In 2002, the first 3.6 tonne, 17 kW hydrogen (fuel cell) -powered mining locomotive was demonstrated in Val-d'Or, Quebec. In 2007 the educational mini-hydrail in Kaohsiung, Taiwan went into service. The Railpower GG20B finally is another example of a fuel cell-electric locomotive.

• Hybrid - A hybrid train is a locomotive, railcar or train that uses an onboard rechargeable energy storage system (RESS), placed between the power source (often a diesel engine prime mover) and the traction transmission system connected to the wheels. Since most diesel locomotives are diesel-electric, they have all the components of a series hybrid transmission except the storage battery, making this a relatively simple prospect.

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