However it was found that the additional number of passengers meant longer time spent at stations to allow passengers to get on and off. The intention to carry more passengers in a train of the same length as other contemporary stock was met, with the two 4-DD units having 1,104 seats compared to 800 in other units of similar age. A mock-up was displayed at London Marylebone in 1949 shortly before it was first introduced into service, 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. 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. 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. 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. The SR Class 4DD was an experimental double-decker electric multiple unit built in 1949 and operated by the Southern Railway until 1971.
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