Conceived by Oliver Bulleid for the Southern Railway's Dartford commuter route, the two 4DDelectric multiple units were the only double-deck trains 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.