A steam tricycle was an early motorized vehicle. Several experimenters built steam powered vehicles with three wheels.
The first steam tricycle, and the first true self-propelled land vehicle was built by Nicolas-Joseph Cugnot in France in 1769. It only went 2 miles per hour, and did not meet army standards so it was never further developed. George A Long built a 2-cylinder steam tricycle in the United States, over a century later, in 1880. The Parkyns-Bateman steam tricycle used petroleum as fuel, and was demonstrated in England in 1881.
Two further steam tricycles were built in France in 1887, one by Jules-Albert de Dion, and the other by Léon Serpollet. Serpollet would later perfect the flash boiler, which made steam a much more practical source of power for an automobile.
On June 12, 1893, after Doc Brown's first experiment with the Jules Verne Train failed, Clara Clayton noticed a man driving a Serpollet steam tricycle. Upon seeing the vehicle, Doc remembered the early experiments with steam powered automobiles, and decided to use Serpollet's innovations in his own steam engine design.