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.

Formations

The history of these units is very complex. Many of the 3Sub units were rebuilt from former locomotive-hauled stock or the AC electric units built by the London, Brighton and South Coast Railway. They were then further converted to 4Sub units during the 1940s (the last 3Sub formation ran in 1949) by the addition of a new 'Augmentation' trailer car and renumbered. The table below illustrates the basic details of units within the broad number ranges used:

Preservation

A handful of Sub stock carriages have survived in preservation, thus:

• Driving Motor car number 8143 (formerly of Sub unit 1293, later 4Sub 4308) is in the care of the National Railway Museum, York.
• 4Sub unit number 4732, formed of DMBTO 12795, TT 10239, TTO 12354 and DMBTO 12796, was retained by British Rail after withdrawal from normal passenger service for use on special occasions. It is currently undergoing restoration at the Coventry Railway Centre, after spending many years stored in the open at various locations.