truck, automotive vehicle designed primarily for the transportation of goods. A truck is constructed on the general lines of the automobile but uses larger and heavier parts. It may be powered by a gasoline internal-combustion engine or a diesel engine. In some trucks propulsion is supplied through a single front or rear axle, in others through two rear axles, and in still others through both front and rear axles. Many trucks have automatic or semiautomatic transmissions. Smaller trucks are built as a single unit, but larger trucks are frequently combinations of a truck tractor, which contains an engine, transmission, and cab, and a semitrailer, which is a trailer that the tractor hauls. The semitrailer has no forward axle, so that its front end must be supported by a swivel mount, known as the fifth wheel, which is found on the rear of the truck tractor. A full trailer, which can be attached to the rear of a semitrailer, has a front axle and one or two rear axles. In other countries, such as Australia, as many as three trailers may be attached to a single tractor. In the United States most states place restrictions on the length of trucks, on the maximum weight that may be carried on a single axle, and on the addition of trailers, though some states still allow up to three trailers. Despite these restrictions, truck traffic accounts for ever-larger percentages of accidents and road damage. As common carriers, motor trucks have made serious inroads on the earnings of the railroads as they carry freight over increasingly long distances. In Asia and Africa, they have replaced the camel caravan and human carriers.