During 1948, in the months prior to nationalisation, O. V. S. Bulleid had plans drawn up for a double-deck or DD unit, based on the successful SUB design. The experimental train was to have accommodation on two levels, but unlike bus seats, were to be arranged in a 'high' 'low' configuration throughout the unit, access to the 'high' deck being by shallow steps.
Design plans were accepted, and the first complete unit emerged late in 1949. Once the four car set was operational, passenger reaction was gauged, and unfortunately this was unfavourable, as passengers soon filled the lower deck, and some even stood, whilst seats remained empty on the upper stage. The second of these units appeared early in 1950, and from then on the two sets usually operated together on the South Eastern Section of the Southern Region on routes from Charing Cross to Dartford.
Each set had accommodation for 552 passengers, with room for about another 150 standing, set out as follows: Driving Motor Brake 55 seats 'high' and 55 seats 'low'; Trailer 66 seats 'high' and 78 seats 'low'. In addition, each car had tip-up seats on the 'high' level. Although sets conformed basically to the SUB design, they were slightly larger, and therefore heavily restricted on their operating area. To permit the stock to conform to the loading gauge, no external stepboards were fitted. Power and control equipment was by English Electric, with traction motors mounted on the outer ends of the driving cars. One advancement fitted on DD stock, from new, was strip lighting, improving the interior illumination. Pressure ventilation was also an essential fitting, as no opening windows were fitted on the upper area. When constructed, the sets were painted in BR green livery and numbered 4001-4002. When the sets passed through works, in the late 1960s, they emerged in BR rail blue and, from November 1970 were renumbered 4901-4902, thus vacating their previous number series for then new high density stock.
The DD units remained in service on the Dartford lines until October 1971, when they were withdrawn. Regretfully most vehicles were broken up, but two vehicles have successfully been preserved.