Shunter

A switcher or shunter (Great Britain: shunter; Australia: shunter or yard pilot; United States: switcher, switch engine, or yard goat, except Pennsylvania Railroad: shifter) is a small railroad locomotive intended not for moving trains over long distances but rather for assembling trains ready for a road locomotive to take over, disassembling a train that has been brought in, and generally moving railroad cars around – a process usually known as switching (USA) or shunting (UK). They do this in classification yards (Great Britain: marshalling yards). Switchers may also make short transfer runs and even be the only motive power on branch lines and switching and terminal railroads. The term can also be used to describe the workers operating these engines or engaged in directing shunting operations.

The typical switcher is optimised for its job, being relatively low-powered but with a high starting tractive effort for getting heavy cars rolling quickly. Switchers are geared to produce high torque but are restricted to low top speeds and have small diameter driving wheels. Switchers are rail analogs to tugboats.

US switchers tend to be larger, with bogies to allow them to be used on tight radiuses. European shunters tend to be smaller and more often have fixed axles. They also often maintained coupling rods for longer than other locomotive types, although bogie types have long been used where very heavy loads are involved, such as at steelworks.

Switching is hard work, and heavily used switch engines wear out quickly from the abuse of constant hard contacts with cars and frequent starting and stopping.. Nevertheless, some types (like the Swedish class U) have been remarkably long-lived.

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Wikipedia