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 The stock was introduced in 1951, in sets, on 'Festival of Britain' trains being built through to 1964, when superseded by the Mark II design. The Brake Second Open (BSO) consists of a standard class open passenger saloon, with a centre aisle, a guard's compartment with hand brake and a lockable luggage compartment, and was built between 1955 and 1963 to two diagrams: 183 and 184. 181 examples were built in total, 163 examples of Diag.183 and eighteen Diag.184 examples (which differed in having Commonwealth Bogies, Fluorescent Lighting and a door separating the sixteen non smoking seats from the twenty three smoking seats). The early deliveries went to the ER/NER and WR, with the ScR batch arriving in 1959. Long-lasting, they ran in crimson and cream, chocolate and cream, lined maroon, and blue/grey.