Built to diagram 146, more of these standard third class (SK) coaches were produced than any other design in the history of Britain's railways and were found throughout the network. The first lot was built in 1952 at Eastleigh and the last in 1963 at the BR coach works at York.

The corridor ran along the side of the coach and had doors both sides at each end and in the centre. The centre vestibule split the compartments into two groups of four. Armrests were provided on seats making three positions per seat and providing space for 48 passengers per coach. With armrests raised out of the way, 64 passengers could be accommodated. There were two lavatories, one positioned either side of the corridor at one end of the coach.