Full trailers or semi-trailers do not come with their own power units. Automobile trains composed of tractors belong to the category of automobiles. Semi-trailers are trailers with axles placed behind the vehicle's center of gravity (when the vehicle is evenly loaded) and equipped with couplings that transmit horizontal or vertical forces to the tractor. That is, part of the total weight of the trailer is borne by the tractor. Its characteristics: a vehicle that is unpowered by itself, carried together with the main car, and relies on the main vehicle to drive for traction.