A tractor unit, traction unit, road tractor or prime mover (Australian English) is a heavy-dutyvehicle, usually with a large diesel engine and two or three axles. Many three axle prime movers have the enginedrive through both rear axles for better road grip on bad roads, construction sites etc.
These tractors are mostly designed for the transport of freight of some kind over a significant distance, to pull long semi-trailers (forming a semi-trailer truck) or road trailers. These trailers have also been expanded into so called Road Trains or LCV's (Long Combination Vehicle) in some countries, especially in Australia.
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