"A tractor unit (prime mover, traction unit or rig) is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. ...
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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." - (en.wikipedia.org 21.12.2019)
