The Southern Railway (SR) gave the designation Sub to the wide variety of electric multiple units that were used on inner-suburban workings in the South London area. Originally these units were formed as three-car units, but in the 1940s, all surviving units were increased to four cars by the addition of an 'Augmentation' trailer. New four-car units were also built at this time, and these survived in passenger use until late 1983, by which time British Rail had allocated to them TOPS Class 405.