After nationalization of the railways in 1948 plans were made not only for the design and construction of standard locomotives, but also for standard coaching stock. As with locos, the best features from the four companies were combined, a prototype carriage emerging from Eastleigh works in 1950. Quantity building commenced in 1951 and eventually about 9,000 vehicles had been turned out by BR workshops and private contractors before production ceased in 1964.

Design features incorporated into the 'Mk1' stock included buckeye couplers and bow ends from the LNER, body-size and outline from the SR Bullied stock, plus the electrical system, window style and body construction from the LMS. Also used were Pullman gangways, a feature of both LNER and SR stock.

The body construction was quite revolutionary for the period and consisted of an all-steel box welded to the underframe. This 'semi-monocoque' type of bodyshell was similar to that employed on motor cars, and its use on railways was pioneered by the LMS. Previously all carriage bodies were of either wood or sheet metal over a wooden frame. British Railways also used semi-monocoque construction for suburban stock and Southern Region electric multiple unit vehicles. The latter remaining in production until 1972, and now almost all out of service.

The 'Mk 1s' have worn various liveries throughout their lives including 'Blood and Custard' (red and cream), maroon, Southern Region green and Western Region chocolate and cream. With the introduction of the British Rail corporate image in 1965, blue and grey was chosen as standard. This was carried by many 'Mk 1s' until the end of their life, but with the introduction of regional liveries in the 1980s, several other liveries, such as Network South East, were used. 'Mk 1s' were replaced by 'Mk 2' and 'Mk 3', first on main line work, then progressively on cross-country routes.

Examples at Quainton