class-405-4sub

The BR Class 405 '4SUB' was the Southern Railway and British Railways Southern Region's standard four-car DC electric multiple unit for suburban stopping services on the third-rail 660–750 V DC network, built in various sub-variants between 1925 and 1951 and forming the backbone of the Southern Region's intensive London suburban electric services from the inner stations out to Guildford, Dorking, Horsham, and the many other suburban destinations served by the SR's electrified network. The 4SUB designation referred to the four-car suburban formation: one motor coach, two trailer coaches, and one driving trailer, configured for the turn-round operation of the intensive Southern suburban timetable.

The Southern Railway's decision to electrify its suburban network using the third-rail DC system from 1916 onwards, and to expand this electrification progressively through the 1920s, 1930s, and into the BR era, created one of the densest suburban electric railway networks in the world, with headways of a few minutes on the busiest inner-suburban routes. The 4SUB provided the rolling stock for this network through the middle decades of the twentieth century, giving BR Southern Region passengers a regular and reliable electric service at frequent intervals on routes that ranged from the intensively used Waterloo–Wimbledon and Victoria–Croydon services to the more lightly-used outer-suburban electric lines.

One 4SUB unit, 4732, is preserved and represents this important Southern electric tradition.