Upon nationalisation in 1947 the newly formed British Railways saw the need to standardise their passenger stock, having inherited a patchwork of coaches of all different specifications from each of the big four who were in turn sometimes running a wide variety of stock from their constituent companies. This new coach would have to be perfect for every possible use that the new BR may need it for, which could be tricky when the coach would need to be used on high speed summer boat trains in the south and cold winter services in the north of Scotland.

These coaches would be constructed from 1951 to 1963 with only one significant change made during this time, a swapping of bogies in the last gasps of the Mark 1s build cycle. The Mark 1 coach as it would be designated was designed to be a ‘go anywhere, do anything’ coach utilising the best features from each of the big four coaches the company had inherited. The Mark One coaches were stronger than the designs they replaced, with their introduction being commended in rail safety reports. These coaches would be in stock in one form or another in the UK until 2010, when the Mark One based Class 421 was withdrawn from service on the Lymington Branch Line.