From
a dedicated web siteConceived by OVS Bulleid for the newly nationalised Southern Railway’s commuter route from London Charing Cross to Dartford, the two 4DD (Double Deck) EMUs▸ were the only double-deck trains to run on the main railway network in Britain.
The 4DD Double Deck units were a unique experiment in UK▸ train design, an experiment which carried on in service for over 20 years. The trains were withdrawn in 1971
An experiment ... that didn't really work ... aimed at getting more passengers per hour flowing on the South Eastern lines through London Bridge in peak hours.
Due to the loading gauge limitations in the UK, the upper and lower compartments were "staggered" - with people in the lower compartment sitting below people in the upper compartment. So each (slam) door catered for 22 passengers - 6 on one side and 5 on the other in the lower compartment, with the sixth seat replaced by a steep stairway, then 5 + 6 above. Cramped as I recall, and the big problem was the station dwell time.
As the 4DD units (4001 and 4002 / became 4901 and 4902) failed to solve the capacity problem, expensive engineering in the "10 car scheme" increased platform lengths to 10 carriages from 8 to make the trains longer. I've not seen much official, but as I recall that only helped somewhat as passengers tended to all join towards the rear leaving London (front on the way in) as the platform entrances at Charing Cross, Waterloo, Cannon Street and London Bridge were all at the same end of the platform.
Two driving cars preserved, both in poor condition and now on a restoration site in Kent where some enthusiastic folks are working on them. It would be lovely to see them running again but it would be very different to Charing Cross to Dartford
