Pick one category for the following text. The options are - company, educational institution, artist, athlete, office holder, mean of transportation, building, natural place, village, animal, plant, album, film or written work. SR Class 4DD - Conceived by Oliver Bulleid for the Southern Railway's commuter route from London Charing Cross to Dartford 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.
Mean Of Transportation