Locomotive

A locomotive (from lat. locus motivus) is a railway vehicle that provides the motive power for a train, and has no payload capacity of its own; its sole purpose is to move the train along the tracks. In contrast, many trains feature self-propelled payload-carrying vehicles; these are not normally considered locomotives, and may be referred to as multiple units or railcars; the use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight (see however CargoSprinter). Vehicles which provide the motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars.

Benefits of locomotives

There are many reasons why the motive power for trains has been traditionally isolated in a locomotive, rather than in self-propelled vehicles. These include:

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• Ease of maintenance - it is easier to maintain one locomotive than many self-propelled cars.
• Safety - it is often safer to locate the train's power systems away from passengers. This was particularly the case for the steam locomotive, but still has some relevance.
• Easy replacement of motive power - should the locomotive break down, it is easy to replace it with a new one. Failure of the motive power unit does not require taking the whole train out of service.
• Efficiency - idle trains do not waste expensive motive power resources. Separate locomotives mean that the costly motive power assets can be moved around as needed.
• Obsolescence cycles - separating the motive power from the payload-hauling cars means that either can be replaced without affecting the other. At some times, locomotives have become obsolete when their cars are not, or vice versa.