| Southern Railway 4-DD | |
|---|---|
One of the SR Class 4DD vehicles. |
|
| In service | 1949–1971 |
| Constructed | 1949 |
| Number built | 2 sets (8 cars) |
| Number preserved | 2 DMBTs |
| Number scrapped | 2 DMBTs, all TTs |
| Formation | DMBT-TT-TT-DMBT |
| Fleet numbers | see text |
| Capacity | 552 seated, 150 standing (per set) |
| Operator | British Rail |
| Line(s) served | Charing Cross to Dartford |
| Specifications | |
| Train length | 257 ft 5 in (78.46 m) |
| Width | 9 ft 3 in (2.82 m) |
| Weight | 133 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 |
| Braking system(s) | Electro-pneumatic |
| Coupling system | Screw-link |
| Gauge | 4 ft 8 1⁄2 in (1,435 mm) |
Conceived by Oliver Bulleid for the Southern Railway's Dartford commuter route, 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 Europe and North America the restrictive railway loading gauge in the United Kingdom prohibits normal double-deck cars with two fully separated decks. On the 4DD carriages the upper-level seats were squeezed in between the lower-level seats to ensure that the overall height of the unit was within the clearances necessary to pass through tunnels and under bridges. 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.
Unusually for an "experiment" they lasted in traffic from 1949 to 1971, undergoing routine maintenance and repaints with no hiccups in their life.
Contents |
| 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) |
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.
Marsden, Colin J. (1983). Southern Electric Multiple-Units 1898–1948. Shepperton, Surrey: Ian Allan Limited. pp. 85–88. ISBN 0-7110-1253-9.
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