Diagram 700 carriages ran as a triplet set with a D36 restaurant first open (Comet B17AS) and a D56 restaurant second open (B17CS). They were known as Festival of Britain sets when first introduced and ran on premier services. A large central kitchen with two anthracite stoves, ovens, boilers and sinks catered for the preparation and service of meals, whilst two adjacent pantries coped with hot drinks and snacks. No passenger seating was provided.
All were built at Doncaster and ran on B1 bogies.