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The SR Class 4DD was an experimental double-decker electric multiple unit built in 1949 and operated by the Southern Railway until 1971. Conceived by
Ghostarchive
and th
Wayback Machine
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
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. 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
Bulleid Double Decker Society
1949 British Pathe newsreel footage of the pre-production mockup
{{British Rail EMU 4DD SR 4DD Train-related introductions in 1949 Double-decker EMUs 750 V DC multiple units
Oliver Bulleid
Oliver Vaughan Snell Bulleid CBE (19 September 1882 – 25 April 1970) was a British railway and mechanical engineer best known as the Chief Mechanical Engineer (CME) of the Southern Railway between 1937 and the 1948 nationalisation, ...
for the Southern Railway's commuter line from London Charing Cross
Charing Cross railway station (also known as London Charing Cross) is a central London railway terminus between the Strand and Hungerford Bridge in the City of Westminster. It is the terminus of the South Eastern Main Line to Dover via Ashf ...
to Dartford
Dartford is the principal town in the Borough of Dartford, Kent, England. It is located south-east of Central London and
is situated adjacent to the London Borough of Bexley to its west. To its north, across the Thames estuary, is Thurrock ...
, the two trains 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
A loading gauge is a diagram or physical structure that defines the maximum height and width dimensions in railway vehicles and their loads. Their purpose is to ensure that rail vehicles can pass safely through tunnels and under bridges, and ke ...
in the United Kingdom prohibits normal double-decker trains with two fully separated decks.
History
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,Archived aGhostarchive
and th
Wayback Machine
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 unit
A multiple-unit train or simply multiple unit (MU) is a self-propelled train composed of one or more carriages joined together, which when coupled to another multiple unit can be controlled by a single driver, with multiple-unit train con ...
s.
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. 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
Maintenance may refer to:
Biological science
* Maintenance of an organism
* Maintenance respiration
Non-technical maintenance
* Alimony, also called ''maintenance'' in British English
* Champerty and maintenance, two related legal doct ...
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.
Formations
Preservation
Driving motor cars 13003 and 13004 survive, the former at Kent Locomotives Ltd - Sellindge, Kent, and the latter at theNorthamptonshire Ironstone Railway Trust
The Northamptonshire Ironstone Railway Trust operates a long heritage railway line at Hunsbury Hill, south-west of Northampton. The line is mainly dedicated to freight working, featuring many sharp curves and steep gradients which were typica ...
. 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.
References
Further reading
*External links
Bulleid Double Decker Society
1949 British Pathe newsreel footage of the pre-production mockup
{{British Rail EMU 4DD SR 4DD Train-related introductions in 1949 Double-decker EMUs 750 V DC multiple units