A tractor unit is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. The largest such vehicles are similar to locomotives. 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.