The SR Class 4DD was an experimental double-decker electric multiple unit built in 1949 and operated by the Southern Railway until 1971.

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.

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,[1] 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 units.

[2] In practice, the upper compartments were cramped and poorly ventilated because the upper-level windows were impossible to open due to tight clearance.