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The SR Class 4DD was an experimental double-decker
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
electric multiple unit
An electric multiple unit or EMU is a multiple-unit train consisting of self-propelled carriages using electricity as the motive power. An EMU requires no separate locomotive, as electric traction motors are incorporated within one or a number o ...
built in 1949 and operated by the Southern Region of British Railways
The Southern Region was a region of British Railways from 1948 until 1992 when railways were re-privatised. The region ceased to be an operating unit in its own right in the 1980s. The region covered south London, southern England and the south ...
until 1971. Conceived by Oliver Bulleid
Oliver Vaughan Snell Bulleid Commander of the Order of the British Empire, 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 ( ...
for the Southern Railway's commuter line from London Charing Cross
Charing Cross railway station (also known as London Charing Cross) is a London station group, central London railway terminus between the Strand, London, Strand and Hungerford Bridge in the City of Westminster. It is the terminus of the South ...
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 in ...
, 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 k ...
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 atLondon Marylebone
Marylebone station ( ) is a London station group, Central London railway terminus and connected London Underground station in the Marylebone area of the City of Westminster. On the National Rail network, it is also known as London Marylebone a ...
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 multiple unit (MU)) is a self-propelled train composed of one or more Coach (rail), carriages joined, and where one or more of the carriages have the means of propulsion built in. By contrast, a locomotive-hauled ...
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
The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure and supporting utilities in industrial, business, and residential installat ...
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 the Northamptonshire Ironstone Railway Trust. 13004 was moved to Kent by the Bulleid 4DD group to join 13003 in May 2023. One trailer was also preserved, but it was scrapped when Ashford Steam Centre closed in 1984, in lieu of rent owed. Driving motor cars 13003 and 13004 are now (May 2023) both in the care of Bulleid 4-DD Group, which plans to restore them.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