SR Class 4DD
|Southern Railway 4-DD|
Class 4DD no 4902 in 1972
|Number built||2 sets (8 cars)|
|Number preserved||2 DMBTs|
|Number scrapped||2 DMBTs, all TTs|
|Fleet numbers||see text|
|Capacity||552 seated, 150 standing (per set)|
|Line(s) served||Charing Cross to Dartford|
|Train length||257 ft 5 in (78.46 m)|
|Width||9 ft 3 in (2.82 m)|
|Weight||133 long tons 0 cwt (297,900 lb or 135.1 t)|
|Traction system||English Electric|
|Electric system(s)||750 V DC|
|Current collection method||Third rail|
|Track gauge||4 ft 8 1⁄2 in (1,435 mm)|
Conceived by Oliver Bulleid for the Southern Railway's commuter route from London Charing Cross to Dartford, the two 4DD electric multiple units were the only double-deck trains to run on the main line railway network in Britain. Whilst common in continental Europe and North America the restrictive railway loading gauge in the United Kingdom prohibits normal double-deck cars with two fully separated decks.
More split-level than truly double-deck (as the compartments were alternately high and low, to ensure that the overall height of the unit was within the clearances necessary to pass through tunnels and under bridges). Unusually for an "experiment" they lasted in traffic from 1949 to 1971, undergoing routine maintenance and repaints with no hiccups in their life. Near the end, they were renumbered 4901 and 4902. 4001 and 4002 were reassigned to another pair of prototype southern region 4-car EMUs, the 4-PEPs.
The 4DD was somewhat unsuccessful because the upper level compartments were cramped and poorly ventilated (the upper level windows could not be opened due to tight clearance). The compartments were pressure-ventilated but the equipment proved to be troublesome. Dwell times at stations were lengthened because of the increase in the 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.
|4001 (later 4901)||13001||13501||13502||13002||(built September 1949)|
|4002 (later 4902)||13003||13503||13504||13004||(built October 1949)|
Driving motor cars 13003 and 13004 survive, the former at Kent Locomotives Ltd - Sellinge, Kent, and the latter at the Northamptonshire Ironstone Railway Trust. One trailer was also saved, but has since been scrapped.