A road train consists of a relatively conventional prime mover, but instead of pulling one trailer or semi-trailer, the road train pulls two or more of them.
Australia has the largest and heaviest road-legal vehicles in the world, with some configurations topping out at close to 200 tonnes. The majority are between 80 and 120 tonnes. Two-trailer road trains, or "doubles" are allowed in rural areas of all Australian states, except in two capital cities - Adelaide in South Australia, and Perth in Western Australia. Three trailer road trains (triples) operate in western New South Wales, western Queensland, South Australia, Western Australia and the Northern Territory, with the last three states also allowing AB-Quads (3.5 trailers). Darwin, NT, is the only capital city in the world that will allow triples and quads to within 1 km of the central business district.
Road trains can be unwieldy, and driving and maneuvering them safely without unduly obstructing traffic is only possible because of the sparse traffic and extremely flat and straight terrain through the Australian outback. Strict regulations also apply regarding licensing and driving experience. The multiple dog-trailers are unhooked, the dollys removed and then connected individually to multiple trucks at "assembly" yards when the road train gets close to populated areas.
Road trains are used for transporting all manner of materials, with livestock, fuel, mineral ores and general freight the most common. Their cost-effective transport has played a significant part in the economic development of remote areas, with some communities totally reliant on a regular service.
During 1999 in the town of Merredin, Western Australia made it into the Guinness Book of Records, when Marleys Transport made a successful attempt on the record for the world's longest road train. The record was created when 45 trailers, driven by Greg Marley, weighing 603 metric tons and measuring 610 metres were pulled by a Kenworth truck for 8 km.
In 2003, the record was surpassed near Mungindi, New South Wales, by a road train consisting of 87 trailers and a single prime mover (measuring 1235 metres in length).
The next record was 1,442 metres, set by a driver in Kalgoorlie, Western Australia in a Kenworth owned by Doug Gould.
In 2006, a truck with 112 semi-trailers (at a length of 1,474.3 metres) claimed a new record at Clifton, Queensland.
That is a great idea and in fact has been in people's minds for quite some time now. Europeans would like to see that done "on-demand" for cars on those super highways called "Autobahn".
As you enter the Autobahn, your car would automatically connect to the first "road train" it encounters, and from then on you don't have to drive it anymore. Volkswagen's experiments with cars that don't need a driver (they are controlled by sensors and a computer) is part of these ongoing investigations.
I guess that in the near future you would just type in your destination in your GPS equipment, and then you might as well go to sleep in your car. The computer would do the driving for you. - I for one would love it!