Tom Thumb was the first American-built steam locomotive to operate on a common-carrier railroad. It was designed and constructed by Peter Cooper in 1829 to convince owners of the newly formed Baltimore and Ohio Railroad (B&O) to use steam engines; it was not intended to enter revenue service. It is especially remembered as a participant in a legendary race with a horse-drawn car, which the horse won after Tom Thumb suffered a mechanical failure. However, the demonstration was successful, and the railroad committed to the use of steam locomotion and held trials in the following year for a working engine.

A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material to heat water in the locomotive's boiler to the point where it becomes gaseous and its volume increases 1,700 times. Functionally, it is a steam engine on wheels.