Construction was at Lancing works in 1949, the first set emerging in September and the second in October. Due to operating restrictions the two units usually worked together as an eight car unit and they were confined to the Dartford routes. The experiment was not a success as they were unpopular with both the operators and passengers. Each unit seated 508, 122 more than the equivalent late series 4 Sub unit. Driving cars had five upper and lower compartments accommodating 55 passengers on each level. The trailer cars seated 66 on the high level within six compartments and on the lower 78 within seven compartments. Also on the high level were tip up seats positioned under the large middle window. The passenger accommodation was cramped and despite the pressure air ventilation the upper levels became uncomfortably hot during warm weather. Also passenger loading and unloading time was extended with 36% more passengers needing to use the same number of doors as a single level unit. Construction costs were also 50% greater than comparable single level units due in part to the need to construct from new rather than re-use underframes and bogies (as was the Southern's custom). Thus the experiment did not lead to more double deck units, with the result being the need to upgrade the infrastructure and run ten car trains, although the two units survived plying commuters between Dartford and the London termini until the autumn of 1971. In their latter days they carried BR overall blue livery and were renumbered 4901/2. The last service they worked was between Charing Cross and Dartford via Bexley on the 1st October 1971. They were then withdrawn and sold for scrap. Two motor coaches from these historic units survive. One is at Hope Farm, Sellindge (near Ashford) - where Southern Locos used to be based, the other (DMBS 13004 from set 4002) is currently being restored at the Northampton Ironstone Railway.

Ironically, in 1999 overcrowding on another part of the Southern - this time commuter services into Waterloo - led to the proposal by South West Trains to introduce double decked trains! If these plans had gone ahead it would have been interesting to see if the current railway management, many of whom do not have a background in railways, would have learned from the lessons of the past.