The Southern Railway (SR) and the British Railways (Southern Region) (BR(S)) used the designation Sub to cover a wide variety of electric multiple units that were used on inner-suburban workings in the South London area. The designation ‘Sub’ was first officially used in 1941 to refer to newly built 4-car units. However, during the 1940s large numbers of earlier ‘3-Car Suburban Sets’ were increased to four cars by the addition of an 'Augmentation' trailer, and became part of the 4-Sub category. The SR and BR (S) continued to build or else rebuild 4-car units to slightly different designs which became part of the 4-Sub Class. Many of these later examples survived in passenger use until late 1983, by which time British Rail had allocated to them TOPS Class 405.

A handful of Sub stock carriages have survived in preservation.