 Originally started as an experiment, British Railways did have the 'Bulleid Double Decker' car SR Class 4DD in operation more than 20 years from 1949 to 1971.As the compartments were alternately high and low, to ensure that the overall height of the unit was within the clearances necessary to pass through tunnels and under bridges, the 'Bulleid Double Decker' was more a split-level than a true double-decker.The 4DD was somewhat unsuccessful because the upper level compartments were cramped and poorly ventilated (the upper level windows could not be opened due to tight clearance).