A road train is little more than a regular semi-trailer tractor truck which is hauling three or more trailers. They are used extensively in Australia and somewhat in Canada to efficiently deliver goods to remote locations across long, reasonably straight stretches of highway. The cost of having one or two drivers use one tractor to pull several trailers is much, much lower than using individual tractors for each one. A road train is often the only economically viable way for goods to reach certain areas without rail access.
A road train can only be used in fairly barren areas with very long stretches of straight freeway with few cities and towns, areas found in abundance in Australia and Canada but rare elsewhere. This is due to their extreme length and poor acceleration and braking power, they're simply not maneuverable enough to operate safely near population centers or in heavy traffic.
The Biggest of the Big Rigs
The tractor, or prime mover, for a road train is under a great deal more stress than one pulling a standard load, so a few modifications are necessary. Most of these trucks are made in the U.S., typically The Mack Heavy Truck Company or Kenworth Motor Truck Company, although to the best of my knowledge road trains are not allowed anywhere in the United States. First and most importantly, the frame of the tractor is reinforced with additional support members to handle the towing load. The wheel base is shorter, for both extra maneuverability and strength. The engine is usually a standard V8 diesel truck engine, but always more than 450HP, with 600HP being more typical. They use, at minimum, 80,000 pound differentials with at least 13 gears and no "lazy axles". All the air brakes for the 3+ trailers require compressed air, so they hold extra air tanks, at least double, to handle this. Finally, since the intention is to haul these huge loads (up to 180 metric tons in the mining industry) across vast distances in the Australian Outback, they are outfitted with extra fuel tanks.
Road trains are used to haul all kinds of goods and materials in bulk, from gasoline to livestock to cars to groceries. They can be box trailers, flatbeds, car carriers, liquid tanks, or just about any other kind of standard semi-trailer. The less dense the loads are, the more trailers can be hauled and the bigger the cost savings. Although most road trains haul 3-4 trailers, they can occasionally haul up to seven. The longest road train ever holds the Guinness record at 1,235 meters (over a kilometer! or 4,052.8 feet, almost a mile!), consisting of 87 trailers hauled by a single tractor. Of course, it didn't move very far and was just a publicity stunt where it was assembled, near Mungindi, New South Wales, Australia on March 29, 2003.
Traffic? What Traffic?
Road trains are usually assembled just outside of major cities, where the materials to be hauled are produced. Regular trucks move the semi-trailers one at a time to the outskirts of town to a road train terminal, where they are hooked together three or more long to the special road train style tractor. From there the road train hauls the goods off the huge distances between cities in the remote Australian Outback or Canadian Wilderness. When they get near cities again, they are broken back up into shorter, 1 or 2 trailer configurations again for safety and maneuverability.
Because they travel through such desolate areas, there is little other traffic on the highways a road train navigates. This is good, because a road train doesn't move at typical Outback highway speeds (typically only about 110 kph, or 60 mph on a highway with no speed limit for regular cars) and they are very difficult to pass due to their length. Their massive weight also makes it impossible to stop quickly. Because of the potential danger to other vehicles, road trains are required to post placards that say "Road Train" on the front and back, and additionally "Long Vehicle" on the back.
Driving long distances with few stops and heavy loads like this is also hard on tires — at least three tires will blow out on a typical road train trip. To help save tires, any emptied flatbeds will be stacked on top of each other for the return trip.
There are a number of hazards to contend with that are unique to such empty areas. Boredom and driver fatigue are particularly devious when traveling 1,000 km (650 miles) per day in 14-hour stretches. Fortunately, many areas in the Outback aren't particularly dangerous to doze off in. There is little to hit in the vast, flat, open areas, and most often a road train driver will simply drive off the road and need to drive back on — after the realization that he is no longer on the road surface wakes him up. Rollovers are uncommon but do happen. Rolling over, running out of fuel, or otherwise breaking down requires a large tow truck capable of hauling big rigs to come by for a rescue, but this could be hundreds of miles from the nearest city and it might be a bit of a wait. Smaller problems can sometimes be handled by mobile workshops that can travel out to handle the minor breakdowns.
Wildlife of course is a constant danger, especially at night, so much so that Australian trucks put bars nicknamed "roo bars" on the cab to help withstand hitting animals. Even a roo bar isn't much protection though when a cow wanders onto the road from one of Australia's many cattle stations that are simply too large to fence in.
The modern (post 1980) road train is a descendent of the road trains used just after World War II. These were nearly identical to their modern counterparts except that they used old US army rigs to haul the trailers, and there were far fewer regulations on what could and could not be done (including a complete lack of weight restrictions). Although the trailers were shorter back then, it wasn't unusual to see the six to eight trailer trains that are much less common today. Since these army rigs weren't designed to haul such huge weights, they could take up to half an hour to reach their full speed of just 50 kph (30 mph). Braking was even worse — the trailers had no air brakes, and the only way to stop was to take your foot off the accelerator and wait.