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 are allowed in rural areas of most of the Australian states. 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 1km of the central Business district.
Overtaking a road train can be quite difficult. Patience, assistance from the driver, and large amounts of clear road are required. Many road train drivers help by blinking their right-turn indicators a couple of times to indicate that the road ahead is clear.
When the flat-top trailers of a road train need to be transported empty, it is common practice to stack them. This is commonly referred to as "doubled-up" or "doubling-up". Sometimes, if many trailers are required to be moved at the one time, they will be tripled stacked, or "tripled-up", as shown in the photo.