A road train or roadtrain is a trucking concept used in remote areas of Argentina, Australia, Mexico, the United States and Canada to move freight efficiently.

The term "road train" is most often used in Australia. In the U.S. and Canada the terms "triples," "turnpike doubles" and "Rocky Mountain doubles" are commonly used for longer combination vehicles (LCVs).A road train consists of a relatively conventional tractor unit, but instead of pulling one trailer or semi-trailer, a road train pulls two or more of them.

Early road trains consisted of traction engines pulling multiple wagons.

During the Crimean War a traction engine was used to pull multiple open trucks. By 1898 steam traction engine trains with up to four wagons were employed in military manoeuvres in England.

In 1900 John Fowler & Co. provided armoured road trains for use by the British forces in the Second Boer War. Lord Kitchener stated that he had around 45 steam road trains at his disposal.

There is an earlier road train built by its inventor in the United Kingdom. It is shown in the No. 320 (No. 8. Vol. 12, February 23, 1907) edition of "The Auto" Title: The Renard Road Train, page 242.

In the 1940s, the Government of South Australia operated a fleet of AEC 8x8 military trucks to transport freight and supplies into the Northern Territory, replacing the Afghan camel trains that had been trekking through the deserts since the late 19th century.

These trucks pulled two or three 6 m (19 ft 8 in) Dyson four-axle self-tracking trailers. With 130 hp (97 kW), the AECs were grossly underpowered by today's standards, and drivers and offsiders routinely froze in winter and sweltered in summer due to the truck's open cab design and the position of the engine radiator, with its 1.5 m (4 ft 11 in) cooling fan, behind the seats.

Australian Kurt Johansson is recognised as the inventor of the modern road train. After transporting stud bulls 200 mi (320 km) to an outback property, Johansson was challenged to build a truck to carry 100 head of cattle instead of the original load of 20. Provided with financing of a couple of thousand pounds and inspired by the tracking abilities of the Government roadtrain, Johansson began construction. Two years later his first road train was running.

Johansson's first road train consisted of a U.S. Army World War II surplus Diamond-T tank carrier, nicknamed "Bertha", and two home-built self-tracking trailers. Both wheel sets on each trailer could steer, and therefore could negotiate the tight and narrow tracks and creek crossings that existed throughout Central Australia in the earlier part of last century.

Freighter Trailers in Australia viewed this improved invention and went on to build self-tracking trailers for Kurt and other customers, and went on to become innovators in transport machinery for Australia.

This first example of the modern road train, along with the AEC Government Roadtrain, forms part of the huge collection at the National Road Transport Hall of Fame in Alice Springs, Northern Territory.

Australia has the largest and heaviest road-legal vehicles in the world, with some configurations topping out at close to 200 tonnes (197 long tons; 220 short tons). The majority are between 80 and 120 t (79 and 118 long tons; 88 and 132 short tons).

Double (two-trailer) road train combinations are allowed in most areas of Australia, and within the environs (albeit limited) of Adelaide, South Australia and Perth, Western Australia. A double road train should not be confused with a B-double, which are allowed access to most of the country and in all major cities.

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 (3.5 trailers).

Darwin is the only capital city in the world that triples and quads are allowed to within 1 km (0.62 mi) of the central business district (CBD). Tasmania and Victoria do not allow the operation of roadtrains on any of their roads. Victoria have previously allowed double road trains to operate around Mildura for the vintage grape harvest.

In Canada, road trains are more commonly referred to as Long Combination Vehicles (LCVs), as Extended Length Vehicles (ELVs), or Energy Efficient Motor Vehicles (EEMVs).Four types of LCV are permitted; turnpike doubles, triples, rocky mountain doubles, and queen city triples.

Turnpike doubles consist of a tractor unit pulling a semi-trailer (up to 53 feet (16.2 m) long). An A-type or C-type converter is connected to the rear of the trailer, and carries a second trailer.

Alternatively, the lead trailer may have a hideaway fifth wheel, which enables direct coupling of the second trailer without a converter. The total permissible length is 38 m (125 ft).

Triples may be up to 35 m (115 ft) in length when using A or C converters, or 38 m (125 ft) in B-train configuration.Rocky mountain doubles are limited to 31 m (102 ft) in overall length, but have the advantage of being legal on two-lane, undivided roads.

A, B and C-train variants are used. Other LCVs may only be used on divided highways.Queen city triples consist of a tractor unit pulling one semi-trailer up to 53 feet (16.2 m) long and two shorter "pup" trailers up to 32 feet (9.8 m) long. Queen city triples are only permitted between the cities of Saskatoon and Regina Saskatchewan. These are the longest combinations allowed in North America on public highways.

In the United States, trucks on public roads are limited to two trailers (two 28 ft (8.5 m) and a dolly to connect.

The limit is 63 ft (19.2 m) end to end). Some states allow three trailers, although triples are usually restricted to less populous states such as Idaho, Oregon and Montana, plus the Ohio Turnpike and Indiana East-West Toll Road.

Triples are used for long-distance less-than-truckload freight hauling (in which case the trailers are shorter than a typical single-unit trailer) or resource hauling in the interior west (such as ore or aggregate).

Triples are sometimes marked with "LONG LOAD" banners both front and rear. "Turnpike doubles"  tractors towing two full-length trailers are allowed on the New York Thruway and Massachusetts Turnpike (Route 90 Boston to Seattle, WA), as well as the Ohio and Indiana toll roads. The term "road train" is not commonly used in the US.