One is designated for the tractor unit, and another for the following trailer.

They are only mobile when connected to the correct tractor unit, which can limit their use in recovery.

These are for tractor units, not straight, rigid, box or other heavy vehicles.

Basically they're just large caravans we link up to a tractor unit.

This now made the trailer and tractor one unit.

In other countries, the tractor unit of each truck is not carried on the train, only the trailer.

Even the tractor unit has been painted to match the container.

A tractor unit can sometimes have a mounted crane located behind the cab.

Several trailers might be towed by one or more tractor units.

The tractor unit has an engine and the semi-trailer does not.