Tractor unit of a truck
A tractor unit (prime mover or traction unit) is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. These fall into two categories: heavy and medium duty military and commercial rear-wheel drive "semi tractors" used for hauling semi-trailers, and very heavy-duty typically off-road capable, often 6×6, military and commercial tractor units, including ballast tractors.
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