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SR Class 4DD

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Title: SR Class 4DD  
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Subject: List of British Rail electric multiple unit classes, Passenger car (rail), British Rail Class 418, Mersey Railway electric units, Liverpool Overhead Railway electric units
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SR Class 4DD

A model of the SR Class 4DD

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.

Design

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.

Disadvantages

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.

Formations

Unit Numbers DMBT TT TT DMBT Notes
4001 (later 4901) 13001 13501 13502 13002 (built September 1949)
4002 (later 4902) 13003 13503 13504 13004 (built October 1949)

Preservation

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.

References

External links

  • Bulleid Double Decker Society
  • 1949 British Pathe newsreel footage of the pre-production mockup
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