SR Class 4DD
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Conceived by Oliver Bulleid for the Southern Railway's Dartford commuter route, the two 4DD electric multiple units were the only double-deck trains to run on the main line railway network in Britain. The upper-level seats were squeezed in between the lower-level seats to ensure that the overall height of the unit was within the clearances necessary to pass through tunnels and under bridges. The 4DD was somewhat unsuccessful because the upper level compartments were cramped and poorly ventilated (the upper level windows could not be opened). Also dwell times at stations were increased because of the increased number of passengers per door. To obtain the extra seating capacity that was being sought, it was instead decided to lengthen trains from eight cars to ten.
As an "experiment" they lasted in traffic from 1949 to 1971 undergoing routine maintenance and repaints with a no hiccups in their life, not bad for prototypes.
[edit] Formations
Unit Numbers DMBT TT TT DMBT 4001 (later 4901) 13001 + 13501 + 13502 + 13002 (built September 1949) 4002 (later 4902) 13003 + 13503 + 13504 + 13004 (built October 1949)
[edit] Preservation
Driving motor cars 13003 and 13004 survive, the former at a private location in Ashford, Kent, and the latter at the Northamptonshire Ironstone Railway Trust. One trailer was also saved, but has since been scrapped.
