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.

Oliver Vaughan Snell Bulleid CBE 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, developing many well-known locomotives.