Designed by R.A Riddles (The Mechanical & Engineering Member of the Railway Executive) and aided by E. Pugson (Carriage & Wagon Chief Officer) and E.S Cox (Design Chief Officer), the Mark I carriage had a separate body and underframe.
The standard body length was 64'6" and a width of 8'8", at cantrail height.
The design was used for hauled passenger stock, multiple unit carriages, and non-passenger carrying stock. For passenger stock, construction continued from 1951 to 1963, while multiple units and non-passenger carrying stock continued to be built until 1974.
At the design stage of these coaches care was taken to ensure that passengers could enjoy an unobstructed view from the train; from all seats. Seats were aligned to windows and on the long-distance design of Mark 1, the windows had a low sill, just 25 inches (635 mm) above the floor.