SR Class 4DD

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Southern Railway 4-DD
Class 4DD no 4902 at Ashford Steam Centre.jpg
Class 4DD no 4902 in 1972
In service1949–1971
Constructed1949
Number built2 sets (8 cars)
Number preserved2 DMBTs
Number scrapped2 DMBTs, all TTs
FormationDMBT-TT-TT-DMBT
Fleet numberssee text
Capacity552 seated, 150 standing (per set)
Operator(s)British Rail
Line(s) servedCharing Cross to Dartford
Specifications
Train length257 ft 5 in (78.46 m)
Width9 ft 3 in (2.82 m)
Weight133 long tons 0 cwt (297,900 lb or 135.1 t)
Traction systemEnglish Electric
Electric system(s)750 V DC
Current collection methodThird rail
Braking system(s)Electro-pneumatic
Coupling systemScrew-link
Track gauge4 ft 8+12 in (1,435 mm)
A model of the SR Class 4DD

Conceived by Oliver Bulleid for the Southern Railway's commuter line from London Charing Cross to Dartford, the two 4DD electric multiple units were the only double-decker trains to be used on the mainline railway network in Britain. Whilst commonly used in continental Europe and North America, the restrictive railway loading gauge in the United Kingdom prohibits normal double-decker trains with two fully separated decks.

History[edit]

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 train was exactly within the clearances necessary to safely pass through tunnels and under bridges. A mock-up was displayed at London Marylebone in 1949 shortly before it was first introduced into service,[1] but an assessment after one year in service revealed that the design would not be the optimum solution to the problems of overcrowding, nor would it help increase capacity, and the decision was made to extend trains from eight coaches to ten coaches by using regular, single-decker multiple units.[2]

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 1,104 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.[2] In practice, the upper compartments were cramped and poorly ventilated because the upper-level windows were impossible to open due to tight clearance. To resolve this problem, the compartments were pressure-ventilated, but the equipment proved to be unreliable.

Near the end, they were renumbered 4901 and 4902. 4001 and 4002 were reassigned to another pair of prototype Southern Region four-coach 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 travelled approximately 700,000 miles in service.[2]

Formations[edit]

Key: Scrapped Preserved
Unit Numbers DMBT TT TT DMBT Notes
Initial
number
Final
Number
4001 4901 13001 13501 13502 13002 (built September 1949)
4002 4902 13003 13503 13504 13004 (built October 1949)

Preservation[edit]

Driving motor cars 13003 and 13004 survive, the former at Kent Locomotives Ltd - Sellindge, Kent,[3] and the latter at the Northamptonshire Ironstone Railway Trust.[4] One trailer was also preserved, but it was scrapped when Ashford Steam Centre closed in 1984, in lieu of rent owed.[5] Driving motor car 13003 is now in the care of Bulleid 4-DD Double Deck EMU Group, which plans to restore it.[3]

References[edit]

  1. ^ https://youtube.com/watch?v=1a2K0TP1lxM
  2. ^ a b c "Double-decker to go". News of the month. Railway World. Vol. 32 no. 378. Shepperton: Ian Allan. November 1971. p. 467.
  3. ^ a b "SR 13003 4-DD 'Double Decker' Driving Motor Brake Third built 1949". Vintage Carriages Trust. Retrieved 1 November 2017.
  4. ^ "SR 13004 4-DD 'Double Decker' Driving Motor Brake Third built 1949". Vintage Carriages Trust. Retrieved 1 November 2017.
  5. ^ Bright, Thomas (3 November 2017). "'O1' saviour Esmond Lewis-Evans dies". Steam Railway. Peterborough: Bauer Consumer Media (457): 18. ISSN 0143-7232.

Marsden, Colin J. (1983). Southern Electric Multiple-Units 1898–1948. Shepperton, Surrey: Ian Allan Limited. pp. 85–88. ISBN 0-7110-1253-9.

Further reading[edit]

  • "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.

External links[edit]