A Road Train is an ITS concept for an efficient platoon of cars drafting on a leader. In one concept, SARTRE , a professional driver would drive the lead vehicle and following vehicles would use electronics to join up and automatically follow close behind. The essential systems are very simple: a tracking device would take over steering, throttle and braking to follow a special device on the car in front. This could be done with optics, radar, or sonar.
Because the cars are drafting closely, they are using much less energy than they would if travelling singly. Because all the cars are effectively controlled by the leader, they are much less likely to get into accidents, especially those caused by inattention or poor judgement. For this same reason the drivers behind can safely occupy themselves with things that would normally be too dangerous while driving: reading, watching video, sleeping, etc.
This is a very old idea, but for the first time, a major body (the EU) is doing a serious study. There are many details that need to be worked out, especially in the area of safety.
The term is also used for trucks towing multiple trailers, especially in Australia