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A road train is a specially-equipped semi-tractor and trailer rig used in Mexico, Argentina, America and Canada, but it is most common in Australia. Typically consisting of a very powerful tractor pulling four full-sized trailers, the road train is considered to be one of the largest wheeled vehicles in the world. Some of the largest road train vehicles operate solely on private land instead of public roads; therefore, weight and length restrictions do not apply. While fuel tankers represent the majority of road train vehicles, there is nearly every type of trailer imaginable in use with the behemoths today.
The earliest configuration for a road train was that of a military truck hauling two or three trailers. These vehicles were not very dependable due to their age. Drivers of these vehicles were often subjected to harsh conditions, as the vehicles had open cabs. The Australian government had commissioned several self-tracking trailers to be built to aid in the maneuverability of the road train. The self-tracking trailer had steerable wheels at each end to aid in maneuvering on the winding roads found in the Australian Outback.
An Australian cattle hauler is credited with the design of the modern road train. Using a surplus United States military truck and several of the self-tracking trailers as prototypes, he built a road train that could haul over 100 bulls 200 miles (320 km) from southern Australia to Northern Australia. Local trailer manufacturers soon began to copy the new trailer design and began producing the self-tracking trailers for several customers. Popular semi manufacturers began producing specially built units, with Kenworth and Mack becoming two of the more preferred models.
While the common units utilize three or four trailers, some of the longer trains will haul up to six trailers at a time. These super trucks are known as Powertrain units. The larger trains use an engine mounted in the back of the rear-most trailer to aid the train in moving. Without the assistance of the rear engine pushing the train from behind, the tractor could break a drive shaft or pull a hitch loose from one of the trailers. The rear engine is operated via remote control from the cab of the road train.
The world's record for the longest train has switched hands a number of times since the 1970s. The record is 112 trailers pulled by a single truck. The record-breaking truck was 4,836 feet, 11 inches (1,474.3 meters) long and was pulled a distance of 328 feet (100 meters).