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 .
Conceived by Oliver Bulleid for the Southern Railway 's Dartford commuter route , the two 4DD electric multiple units were the only double-deck trains to run on the main line railway network in Britain .