A switcher locomotive (American English), shunter locomotive (British English), station pilot (British English), or shifter locomotive (Pennsylvania Railroad terminology) is a locomotive used for maneuvering railway vehicles over short distances. Switchers do not usually move trains over long distances. Instead, they typically assemble trains in order for another locomotive to take over. Switchers often operate in a railyard or make short transfer runs. They may serve as the primary motive power on short branch lines or switching and terminal railroads. A hybrid type known as a road switcher can both shunt and haul trains.

Switchers are optimized for their role, 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 tend to be durable and to remain in service for a long time, such as the Swedish class U.

American, Russian, Indian and Chinese switchers tend to be larger, with bogies to allow them to be used on tight radiuses. Western European shunters tend to be smaller and more often have fixed axles^{[citation needed]}. 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.

A switcher may also be called a yard pilot, switch engine, or yard goat.

The term can also be used to describe the workers operating these engines or engaged in directing shunting operations. Switching locomotives may be purpose-built engines, but may also be downgraded main-line engines, or simply main-line engines assigned to switching. Switchers can also be used on short excursion train rides.

Diesel switchers tend to have a high cab and often lower and/or narrower hoods (bonnets) containing the diesel engines, for all round visibility. Slugs are often used because they allow even greater tractive effort to be applied. Nearly all slugs used for switching are of the low hood, cabless variety. Good visibility in both directions is critical, because a switcher may be running in either direction; turning the locomotive is time-consuming. Some earlier diesel switchers used cow–calf configurations of two powered units in order to provide greater power.

Modern diesel switchers are usually diesel-electric locomotives.

The majority of modern switchers are diesels, but countries with near-total electrification, like Switzerland, use electric switchers. Prior to the introduction of diesel-electric locomotives, electric shunting locomotives were used to an extent in Great Britain where heavy trains needed to be started on steep gradients. The steeply-graded Quayside Branch in Newcastle upon Tyne was electrified by the North Eastern Railway in 1905, and two steeplecab locomotives were built to handle all traffic on the line. One of these, No. 1, is now part of the National Collection and resides at Locomotion in Shildon. On the opposite side of the Tyne, the electrified lines owned by the Harton Coal Company in South Shields for the movement of coal and colliery waste to shipping facilities on the river was one of the more extensive industrial networks. A number of the early German locomotives built for use on these lines have been preserved.