A tractor unit is a large vehicle that can be used to tow trailers, usually with a fifth wheel hitch. These large trucks can have anywhere between two and five axles, two or more of which may be used for steering or moving the vehicle. Unlike rigid vehicles, a tractor unit can decouple from one trailer and hook up another before the first has been unloaded. This modular nature allows for a great deal of versatility when using tractor units to haul freight overland. Tractor units can be used to tow liquid filled tankers, shipping containers, and many other types of trailers.
Tractor units are referred to as prime movers in some countries, and road tractors and traction units are other common names. Another term that can refer to these vehicles is semi-truck because they are often used to tow semi-trailers. When attached to a load, the combined unit is often called a tractor trailer. In Australia, where some of the heaviest and longest configurations are used, tractor trailers are often called road trains. Since it is possible for a tractor unit to haul the same modular shipping containers as freight ships and trains, these vehicles represent a versatile component of the worldwide shipping industry.
In addition to shipping containers, tractor units can also be used to tow a wide variety of other trailers. Liquids, gases, and certain dry goods can be hauled by tractor units in special trailers known as tankers. Some countries require tractor units that haul flammable liquids or gases to have a specially altered electrical system. This usually involves modifying the system so that neither negative or positive is grounded to the frame in order to reduce the possibility of sparks that could ignite the load. Other trailers such as box units, dumpers, and flatbeds can typically be hauled without any modifications.
Most tractor units are powered by diesel engines that have large displacements and can provide sufficient torque to haul heavy loads. If a tractor unit is built to haul especially heavy trailers, it will often have three or more. Some countries and localities have regulations that state how much weight must be distributed to each axle of a vehicle. When a tractor unit has over two axles, it is common for more than one of them to be driven. Some tractor units with three or more axles have an additional steering axle, which may also be driven as well.