What is SR Class 4DD?
[[Image:Class 4DD double decker coach.jpg|thumb|One of the SR Class 4DD vehicles]] [[Image:SR Class 4DD Model.jpg|thumb|A model of the SR Class 4DD]] Conceived by [[Oliver Bulleid]] for the [[Southern Railway (Great Britain)|Southern Railway]]'s [[Dartford]] commuter route, the two '''4DD''' [[electric multiple unit]]s were the only [[Bilevel car|double-deck train]]s to run on the main line railway network in Britain. The upper-level seats were squeezed in between the lower-level seats to ensure that the overall height of the unit was within the clearances necessary to pass through tunnels and under bridges. 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). The compartments were pressure-ventilated but the equipment proved to be troublesome. Also dwell times at stations were increased because of the increased number of passengers per door. To obtain the extra [[seating capacity]] that was being sought, it was instead decided to lengthen trains from eight cars to ten. {{British Rail EMU}} {{transport-stub}} [[Category:British Rail electric multiple units|SR 4DD]]