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.
The 4DD was more split-level than truly double-deck because 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. A mock-up was displayed at London Marylebone in 1949 shortly before it first ran in service, but an assessment after one year in service revealed that the design would not resolve the problems of overcrowding or increase the capacity, and the decision was taken to lengthen trains from eight cars to ten by using regular, single-decker multiple units.
The intention to carry more passengers in a train of the same length as other contemporary stock was met, with the two 4-DD units having 1104 seats compared to 800 in other units of similar age. However it was found that the additional number of passengers meant longer time spent at stations to allow passengers to get on and off. In practice the upper compartments were cramped and poorly ventilated with the upper level windows impossible to open due to tight clearance. To resolve this the compartments were pressure-ventilated but the equipment proved to be troublesome.
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.
Unusually for an "experiment" they lasted in traffic from 1949 to 1971, undergoing routine maintenance and repaints with no hiccups in their life. The two units were finally withdrawn on 1 October 1971 having covered approximately 700,000 miles in service.
|4001||4901||13001||13501||13502||13002||(built September 1949)|
|4002||4902||13003||13503||13504||13004||(built October 1949)|
Driving motor cars 13003 and 13004 survive, the former at Kent Locomotives Ltd - Sellindge, Kent, and the latter at the Northamptonshire Ironstone Railway Trust. One trailer was also preserved, but it was scrapped when Ashford Steam Centre closed in 1984, in lieu of rent owed. Driving motor car 13003 is now in the care of Bulleid 4-DD Double Deck EMU Group, which plans to restore it.
- "Double-decker to go". News of the month. Railway World. Vol. 32 no. 378. Shepperton: Ian Allan. November 1971. p. 467.
- "SR 13003 4-DD 'Double Decker' Driving Motor Brake Third built 1949". Vintage Carriages Trust. Retrieved 1 November 2017.
- "SR 13004 4-DD 'Double Decker' Driving Motor Brake Third built 1949". Vintage Carriages Trust. Retrieved 1 November 2017.
- Bright, Thomas (3 November 2017). "'O1' saviour Esmond Lewis-Evans dies". Steam Railway. Peterborough: Bauer Consumer Media (457): 18. ISSN 0143-7232.
- "Cash plea for Double-Decker EMU's 50th birthday rebuild". RAIL. No. 345. EMAP Apex Publications. 2–15 December 1998. p. 63. ISSN 0953-4563. OCLC 49953699.