Van trailer, Box trailer

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Semi-trailer or Van trailer, Box trailer
 
A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, a detachable front axle assembly known as a dolly, or the tail of another trailer. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled.
A road tractor coupled to a semi-trailer is often called a semi-trailer truck or semi in the US and an articulated lorry or artic in the UK. The fifth wheel on a truck connects to a semi trailer Kingpin. Kingpins come in many guises, however the most common within the UK market is the 2.0" (*0.8mm) EEC approved type. This Kingpin is fully interchangeable and, given a strict maintenance schedule, it should last the life of a trailer.
In Australian English, the tractor unit is usually referred to as a prime-mover; and the combination of a prime-mover and trailer is known as a semi-trailer or semi. Semi-trailers with two trailer units are called B-Doubles (or in American English often just doubles), and in some cases (especially when there are three or more trailers), road trains. A B-double consists of a prime mover towing two semi-trailers, where the first semi-trailer is connected to the prime mover by a fifth wheel coupling and the second semi-trailer is connected to the first semi-trailer by a fifth wheel coupling. A road train means a combination, other than a B-Double, consisting of a motor vehicle towing at least two trailers (counting as a single trailer a converter dolly supporting a semi-trailer)