Prime mover (tractor unit) in the context of "Agricultural engine"

A traction engine is a steam-powered tractor used to move heavy loads on roads, plough ground or to provide power at a chosen location. 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 also heavy, slow, and difficult to manoeuvre. Nevertheless, they revolutionized agriculture and road haulage at a time when the only alternative prime mover was the draught horse.

T-20 armored tractor Komsomolets (Bronirovannyy gusenichnyy tyagach Komsomolets T-20), an armored continuous track tractor, the T-20 was a prime mover vehicle used by the Soviet Union during the Winter War and World War II.