 thumb|a model of the 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. ==design== more split-level than truly double-deck (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). unusually for an "experiment" they lasted in traffic from 1949 to 1971, undergoing routine maintenance and repaints with no hiccups in their life. near the end, they were renumbered 4901 and 4902. 4001 and 4002 were reassigned to another pair of prototype southern region 4-car emus, the 4-peps. ==disadvantages== 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. dwell times at stations were lengthened because of the increase in the number of passengers per door. to obtain the extra seating capacity that was being sought, it w
British_Rail_electric_multiple_units Double-decker_rail_vehicles Southern_Railway_(Great_Britain)_electric_multiple_units