The locomotive is a powerhouse mounted on a frame that produces the motive power needed for traction on railways. There are three distinct locomotives used on the railways, each drawing its power from a different energy source. In a steam locomotive, the motive power is the steam generated in a pressure vessel called the boiler. Thus the thermal energy of fuel is converted into the mechanical energy of motion. In a diesel locomotive, the motive power is an internal combustion engine, which uses high-speed diesel oil as its source of energy. An electric locomotive derives its power from an electric conductor running along the track.