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A semi-trailer truck (colloquially known as an '18-wheeler', 'tractor trailer' or 'big-rig' in the US, and as a 'semi' in Australia, US, and Canada) is an articulated truck consisting of a towing engine ("tractor" in the US), and a trailer that carries the freight. (See below for the etymology of the name "semi-trailer").

Semi tractors usually have 3 axles, the front, or "steer" axle having two wheels, and each of the two rear "drive" axles having a pair of "dual" (double) wheels on each side. Thus, the most common configuration of tractor has 10 wheels. The cargo trailer usually has two axles at the rear, each of which has dual wheels, or 8 wheels on the trailer. The entire vehicle thus usually has 18 wheels in total, although the trailers can vary in number of wheels. Overall lengths often range from 50 to 70 feet in the US, and most states limit the overall weight to 80,000 pounds. The long-haul towing engines used in interstate travel are often equipped with a "sleeper" behind the driver's cab, which can be anything from a small bunk to a rather elaborate miniature apartment.

Most semis use air rather than hydraulic fluid to actuate the brakes. If the air lines fail and the pressure drops during driving, the trailer brakes will actuate and stay applied, causing the wheels of the trailer to lock and skid on the road. Because of the wide variety of loads the semi may carry, they usually have a manual transmission to allow the driver to have as much control as possible. A special driver's license is required to operate a semi-trailer in most countries.

The cargo trailer is hooked to a coupling device called a "5th wheel" at the rear of the towing engine which allows easy hook up and release. The trailer cannot move by itself because it only has wheels at the rear end, hence the name semi-trailer: it only carries half its own weight. The vehicle has a tendency to fold at the pivot point between the semi and the trailer when braking hard at high speeds. Such a truck accident is appropriately called a jack-knife, or jack-knifing.

Modern day semi-trailer trucks often operate as a part of an international transportation infrastructure to support containerized cargo shipment. Some flat bed train cars are modified to hold the cargo trailer with wheels and all. This is called "piggy-back" in North America. The system allows the cargo to switch from the highway to railway or vice versa with ease.

The large trailers pulled by a semi come in many styles, lengths, and shapes. Some common types are: vans, reiffers, flatbeds, and tankers. These trailers may be refrigerated, heated, ventilated, or pressurized, depending on climate and cargo. Some trailers have movable wheel axles that can be adjusted by moving them on a track underneath the trailer body and securing them in place with large pins (thick unthreaded bolts). The purpose of this is to help adjust weight distribution over the various axles, to comply with local laws.

On some interstate highways in the US, long-haul semi-trailer trucks can tow another full trailer at the end, which makes the vehicle look like a two-car small train. Some of the second cars are full trailers with wheels on both ends, while others are just regular semi-trailer cars hooked to the standard coupling device on another set of wheels in tow. There are proposals to allow a third car to be added to the vehicle, but they face strong objections from some car drivers who share the highway with these longer trucks.

In Australia, semi-trailers with more than one trailer are known as road trains.

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