A traction engine is a Victorian-time self-propelled steam engine used to move heavy loads on roads, plow ground or to provide power at a chosen location. They became popular in industrialized countries from around 1850, when the first self-propelled portable steam engines for agricultural use were developed. Production continued well into the early part of the 20th century when competition from internal combustion engine-powered tractors saw them fall out of favor, although some continued in commercial use in the United Kingdom well into the 1950s and later.
The name derives from the Latin tractus, meaning 'drawn', since the prime function of any traction engine is to draw a load behind it. They are sometimes called road locomotives to distinguish them from railway locomotives – that is, steam engines that run on rails.
Traction engines tend to be large, robust and powerful, but heavy, slow, and difficult to maneuver. Nevertheless, they revolutionized agriculture and road haulage at a time when the only alternative prime mover was the draught horse.
Several thousand examples have been preserved worldwide, many in working order. Steam fairs are held throughout the year in the United Kingdom, and in other countries, where visitors can experience working traction engines at close hand.