| Southern Railway 4-DD | |
|---|---|
 Class 4DD no 4902 in 1972 | |
| 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) |
| Operators | British Rail |
| Lines 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 long tons 0 cwt (297,900 lb or 135.1 t) |
| Traction system | English Electric |
| Electric system(s) | 750 V DC |
| Current collector(s) | Third rail |
| Braking system(s) | Electro-pneumatic |
| Coupling system | Screw-link |
| Track gauge | 4 ft 8+1⁄2 in (1,435 mm) |
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 Oliver Bulleid for the Southern Railway's commuter line from London Charing Cross to Dartford, 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 in the United Kingdom prohibits normal double-decker trains with two fully separated decks.