Locomotives, vehicles that run on rails and are self-propelled by any of several forms of energy for the purpose of moving railroad cars, were developed early in the 19th century. The first steam locomotive was built in 1804 by Richard Trevithick for the Penydarren Iron Works in Wales. English locomotives played a role in early American railroad history. U.S. railroads imported more than 100 English locomotives between 1829 and 1841.

By 1830, the development of American locomotive designs was well underway, including many innovations that guaranteed progress for the mode of transport of freight and passengers. John B. Jervis' Experiment of 1832 successfully demonstrated an engine with a single pair of drivers and a four-wheeled swivel truck under the front of the locomotive to allow the engine to negotiate most curves with ease and later proved capable of much higher speeds.

In 1836, Henry Campbell designed an eight-wheeled engine, which dominated U.S. locomotive design for five decades. Three years later, Joseph Harrison's equalizing beam permitted equal pressure by each driver wheel, even on rough track. Isaac Dripps invented the "cowcatcher," placed first on the John Bull, which was necessary in America where few tracks were protected by fencing.

In the half century between the Civil War and World War I, a new emphasis was placed on strength, bulk, and power in the manufacture of American locomotives. Improvements in U.S. steam locomotives continued during the first half of the 20th century. During World War I, the U.S. Railroad Administration purchased 1,930 locomotives, all built to standard specifications.