SR Class 4DD

Southern Railway 4-DD
Class 4DD no 4902 in 1972
In service	1949–1971
Constructed	1949
Number built	2 sets (8 cars)
Number preserved	2 DMBTs
Number scrapped	2 DMBTs, all TTs
Formation	DMBT-TT-TT-DMBT
Fleet numbers	see text
Capacity	552 seated, 150 standing (per set)
Operator(s)	British Rail
Line(s) served	Charing Cross to Dartford
Specifications
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
Braking system(s)	Electro-pneumatic
Coupling system	Screw-link
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.

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. Driving motor car 13003 is now in the care of Bulleid 4-DD Double Deck EMU Group and plans to restore her as an exhibition coach.

Marsden, Colin J. (1983). Southern Electric Multiple-Units 1898–1948. Shepperton, Surrey: Ian Allan Limited. pp. 85–88. ISBN .