HO 40' Trailer, Illinois Central #T-40130 Overview
A semi-trailer is a trailer without a front axle. The are favored in the trucking industry for hauling freight for long distances.
Road tractors and semi-trailers are responsible for carrying a large proportion of cargo. Box or van trailers are the most common type of semi-trailers. They are quite simply a metal box on wheels with some doors on the back, though some offer additional access doors on the sides.
A large proportion of a semi-trailer's weight is supported by a tractor unit, or a detachable front-axle assembly known as a dolly, or the tail of another trailer. The semi-trailer's weight is semi-supported by its own wheels, at the rear of the semi-trailer. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled.