 The trailers can be coupled and uncoupled quickly, allowing them to be shunted for loading and to be trucked between depots.If a power unit fails, another tractor can replace it without disturbing the cargo.Compared with a full trailer, a semi-trailer attached to a tractor unit is easier to reverse, since it has only one turning point (the coupling), whereas a full trailer has two turning points (the coupling and the drawbar attachment).