A tractor unit, traction unit, road tractor or prime mover is a heavy-duty vehicle, usually with a powerful engine and several axles. The tractor unit serves as a method of moving trailers. Trailers can be swapped between tractor units quickly so the unit does not stand idle whilst the load is unloaded or loaded, unlike a rigid, and is not restricted to one type of goods as trailer types can be swapped e.g. Bulk tipper to a box van. The tractor-trailer combination also means that a load can be shared across many axles, yet be more maneuverable than a equivalently sized rigid truck. The tractor unit couples to the trailer using some sort of mechanical lock system, usually a fifth wheel. A tractor unit is primarily designated a freight vehicle and is most common in the heavy goods vehicle class of vehicles although smaller van based tractor units do exist.
A tractor unit was used a haul a trailer from an Insurrectionist bomb assembly factory to a diner on Tribute. The cabin was damaged as a round from a Model 99 Special Application Scoped Rifle punched through it, killing the driver. The trailer was later detonated by his accomplice, destroying the trailer, tractor unit, and diner, killing 38 civilians and three UNSC Marines.
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