Five sets were built at Doncaster. In three sets the RFO was to diagram 12B which differed from 12A in having a 24 seat smoking saloon rather than 18 seats. Diagram 12B featured fixed seating whilst 12A had loose chairs, and clothes cupboards in the end. The kitchen cars were all electric to reduce fire risk. All the coaches had turnbuckle underframes.
The triplets were intended for premier workings and the two D12A sets were decorated in “White Allom” for the Flying Scotsman.