When the Mark 1 coach was introduced in 1951 it was the first new design after nationalisation in 1948.

These 'Mark 1s' eventually formed the mainstay of BR's coaching stock and there are many examples of the various types of coaches in preservation. The Mark 1 coach was the first BR standard design, with a steel chassis, framework and panels. Many thousands were constructed between 1951 and the mid 1960s when they were superseded by Mark 2 stock. Some were built by the various BR works, others by contractors, to increase the rate of production. Each coach has its own number to enable it to be identified for maintenance and service allocation. Each BR Region was allocated its own stock which was denoted by the letter ahead of the number: W- Western, E- Eastern, M- Midland, S- Southern, Sc- Scottish, and for a short while the GE- Great Eastern section was valid, until absorbed into the Eastern region. All coaches have a works plate on the under frame giving the date it was built, the works or contractor responsible, the lot number and the diagram number. On the coach end is a dimension plate, giving overall sizes, number of seats fitted and weight of the vehicle. Coaches were built in various forms with different combinations of internal styles to suit the varying traffic loadings on different routes. The later BR Mark 1 coaches had Formica internal panels instead of the more traditional wood, and fluorescent lighting instead of the tungsten bulbs in earlier versions. Although of standard design, each works, or contractor, who built the various batches, had their own drawing office. They specified the exact dimensions of each component; therefore a window slider from one coach will not necessarily fit another from a different builder. As built the Mark 1s had end steps to allow the station staff to monitor the filling of the water tanks. These were gradually removed for safety as overhead electrification began to spread throughout the railway system. The average cost of a new coach in 1951 was £5,969. By the mid 1960s, when Mark 2 coaches were introduced, the average cost had risen to £16,436