High-speed rail is a type of rail transport that operates significantly faster than traditional rail traffic, using an integrated system of specialized rolling stock and dedicated tracks. While there is no single standard that applies worldwide, new lines in excess of 250 km/h and existing lines in excess of 200 km/h are widely considered to be high-speed, with some extending the definition to include much lower speeds (e.g. 160 km/h) in areas for which these speeds still represent significant improvements. The first such system began operations in Japan in 1964 and was widely known as the bullet train. High-speed trains normally operate on standard gauge tracks of continuously welded rail on grade-separated right-of-way that incorporates a large turning radius in its design.

The InterCity 125 was the brand name of British Rail's High Speed Train (HST) fleet, which was built from 1975 to 1982 and was introduced in 1976. The InterCity 125 train is made up of two Class 43 power cars, one at each end of a fixed formation of Mark 3 carriages (the number of carriages varies by operator). The train operates at speeds of up to in regular service, and has an absolute maximum speed of , making it the fastest diesel-powered train in the world, a record it has held from its introduction to the present day. Initially the sets were classified as Classes 253 and 254. A variant of the power cars operates in Australia as part of the XPT.