A switcher (the general United States usage; common British terminology is shunter, while the Pennsylvania Railroad used shifter) is a small railroad locomotive intended not for moving trains any great distance but rather for assembling a train ready for a road locomotive 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 are either tank locomotives or have special tenderss, with such things as narrow coal bunkers and/or sloped tender decks to increase rearward visibility. Headlights, where carried, were mounted on both ends. Diesel switchers tend to have a high cab and often lower and/or narrower hoods (bonnets) 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.
British and European locomotives of this type tend to be much smaller than the common size in the United States. Almost all European steam switchers were tank locomotives.