Hello; The term road train is used in Australia. In contrast with a more common semi-trailer towing one trailer or semi-trailer, the prime mover of a road train hauls two or more trailers or semi-trailers. Australia has the longest and heaviest road-legal road trains in the world, weighing up to 200 tonnes (197 long tons; 220 short tons). They transport freight across some of the harshest environments on Earth. Double (two-trailer) road train combinations are allowed on some roads in most states of Australia, including specified approaches to the ports and industrial areas of Adelaide, South Australia and Perth, Western Australia. A double road train should not be confused with a B-double, which is allowed access to most of the country and in all major cities. In South Australia, B-triples up to 35.0 metres (114 ft 10 in) and two-trailer road trains to 36.5 metres (119 ft 9 in) were only permitted to travel on a small number of approved routes in the north and west of the state, including access to Adelaide's north-western suburban industrial and export areas such as Port Adelaide, Gillman and Outer Harbour via Salisbury Highway, Port Wakefield Road and Augusta Highway before 2017. A project named Improving Road Transport for the Agriculture Industry added 7,200 kilometres (4,500 mi) of key routes permitted to operate vehicles over 30 m (98 ft 5 in) in 2015–2018. Triple (three-trailer) road trains 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 (B double with two additional trailers coupled behind). Darwin is the only capital city in the world where triples and quads are allowed to within 1 km (0.62 mi) of the central business district (CBD).