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, with a standard body length of 64ft 6in, an underframe 63ft 5in long and bogies at 46ft 6in centres. Body width was eventually settled at 8ft 8in, at cantrail height.
The stock was introduced in 1951, in sets, on 'Festival of Britain' trains being built through to 1964, when superseded by the Mark II design.
