Since it became apparent in the seventies that electrification of all main lines would not be possible, British Railways had a diesel-powered high-speed train built. In order to keep the axle loads low, a streamlined locomotive with a fast-running and light Paxman Valenta with 2,250 hp was used at each end of the train. The trains reached a service speed of 125 mph or 201 km/h and were initially classified as class 253 and 254 multiple units. Only later were they renumbered into class 43 locomotives.

The trains ran with a variable number of Mark 3 coaches and quickly spread across Britain. They are officially simply called “HST” (“High Speed Train”), but were also used as “Intercity 125” in reference to their top speed. They significantly shortened travel times compared to trains with conventional diesel locomotives. With a record of 148.5 mph (239 km/h), they are considered the fastest diesel locomotive in the world.

With the privatization of British Railways, the HST came to a variety of operators. The new VP185 engine was developed before privatization, but was only installed in some of the vehicles. After the turn of the millennium, more were re-engined with new MTU engines. At the same time, some of them were replaced by new multiple units, such as the families of the 220 and 800 classes. However, due to the increased number of passengers, many remained in service. Even after 2020, many HSTs were still active in passenger transport. Today some are used by Network Rail or to ferry trains. Some were even exported to Mexico and Nigeria.