The British Rail Mark 3 is a type of passenger carriage developed in response to growing competition from airlines and the car in the 1970s. A variant of the Mark 3 became the rolling stock for the High Speed Train (HST). Originally conceived as locomotive-hauled coaching stock, the first coaches built were for the prototype HST in 1972. Production coaches entered service between 1975 and 1988, and multiple-unit designs based on the Mark 3 bodyshell continued to be built until the early 1990s. Most of the surviving fleet of the Mark 3 and its derivatives were still in revenue service on the British railway network in 2020, however, as of 7 April 2021, 300 carriages have been sent for scrap. Introduction Under the chairmanship of Stanley Raymond, it was decided to reduce journey times further on long-distance trains by increasing line speed to , where practical – the maximum considered possible on Britain's Victorian-age railways. At the end of 1968, proposals were submitted to the Commercial and Operating Departments of British Rail for a new fleet of third-generation standard coaching stock, designed to run at 125 mph. The rapid development required for the HST in 1969 made the Mark 3 coach design the obvious choice for that train and, in 1972, the first ten Mark 3 coaches were built for the prototype HST. Construction The Mark 3 looks similar to Mark 2D, 2E and 2F coaches, but is of a completely different design. It has a ridged roof and under-frame skirt, compared with a smooth roof and visible below-frame equipment on the Mark 2. The bodyshell is long, almost longer than the Mark 2, of full monocoque construction with an all-welded mild steel stressed skin, and has a reputation for its exceptional strength and crashworthiness. An important advance over its predecessor was the adoption of secondary air suspension between the body and the bogies, giving an exceptionally smooth ride. The bogies, classified BT10 (BT5 on the prototype vehicles), were designed specifically for the Mark 3 and have coil-spring primary suspension with hydraulic dampers, enabling a maximum speed of – the Mark 2 is limited to . Disc brakes in place of the Mark 2 clasp brakes completed the engineering package enabling – in conjunction with wheel slip protection (WSP) – efficient deceleration from 125 mph and almost silent brake operation. Ancillaries, such as electrical and air-conditioning systems, were grouped together