In railroad terminology, a switcher is a small locomotive used not for pulling trains over the road but rather for assembling a train ready for a road unit to take over, disassembling a train that has been brought in, and generally moving railroad cars around. Switchers may also make short transfer runs and even be the only motive power on branch lines.

The typical switcher is optimised for its job, being fairly low-powered but with a high starting tractive effort for getting heavy cars rolling quickly. Top speed is low, and no large-diameter driving wheels are to be found here. Good visibility in both directions is critical, because a switcher may be running in either orientation; there's no time or space to turn a locomotive in a switcher's job. Steam switchers had special tenders, with such things as narrow coal bunkers and sloped tender decks. Headlights were mounted on both ends. Diesel switchers tend to have a high cab and short hoods containing the diesel engines, for all round visibility.

Switching is hard work, and heavily used switch engines wear out quickly from the abuse of constant hard contacts with cars. On the other hand, lightly used switchers last forever; there are even today a number of diesel switchers that predate the second world war still in service.

In England, the word shunter is generally used to describe these locomotives, and European switchers in general were more likely to be tank locomotives in the days of steam. The Pennsylvania Railroad, proud of being different, used the term shifter to refer to these locomotives.

The theme of Apple Computer's latest ad campaign in which a Windows user who moves to the Mac is defined as a 'Switcher'.

Centered around TV and print ads, the multi-million dollar campaign features "real people" from various walks of life who have made the platform jump talking about their experiences. The group of switchers featured in the ads is composed of a mixture of both men and women of all ages. Their vocations include: high school student, Windows Network Administrator, project manager, programmer, producer, publisher, writer, DJ, filmmaker, PR consultant and a designer.

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