A steam locomotive is a locomotive that produces its power through a steam engine. The term locomotive refers to any self-propelled vehicle. This may also include a road locomotive such as a traction engine or steam car, however when the term is used on its own, it commonly refers to a railway locomotive. Its purpose is to haul a train for freight wagons or passenger coaches along a railway track. The locomotive is usually fueled by coal, wood or oil. This fuel is burned to produce steam in a boiler, which drives the steam engine. Both fuel and water supplies are carried with the locomotive, either on the locomotive itself or in wagons pulled behind. Beginning in Britain, steam locomotives dominated railway transportation from the start of the 19th century, until the middle of the 20th century. They were gradually improved and developed in their over 150 years of use. Starting in about 1930, other types of engines were developed, and steam locomotives were gradually overtaken.