A semi-trailer truck is the combination of a powered tractor unit and one or more semi-trailers, joined by a fifth-wheel coupling that lets the tractor carry roughly half the trailer's weight since the trailer itself has no front axle. The type replaced traction engines in commercial and heavy haulage roles mainly between the 1920s and the 1950s, as operators demanded greater efficiency, speed and reliability than earlier haulage methods offered. Design conventions vary by region: European tractors typically use a cab-over-engine layout for compactness, while North American tractors mostly place the engine ahead of the cab in a conventional layout, and maximum weight and length rules differ sharply by jurisdiction, from roughly 40 tonnes under European Union standards to specialized Nordic combinations exceeding 90 tonnes. Today the semi-trailer truck is a central component of global trade infrastructure, especially for moving containerized cargo between highway and intermodal rail networks.
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