The Southern Railway created classification and numbering systems for its large fleet of electric multiple units, perpetuated by the Southern Region of British Rail until the early 1980s, when the impact of TOPS was felt. Some stock is still allocated Southern-style classifications in a semi-official manner.
The early AC electric multiple units were referred to by a two-letter code. This was adapted for the DC third-rail system that was adopted by the Southern Railway, with units being given a three-letter code (sometimes two letters) prefixed by the number of cars in each unit, e.g. 4SUB for a 4-car suburban unit.
The Southern Region perpetuated this, and the same principles were adopted for diesel-electric multiple units but with single-letter codes. The last type to be officially allocated a designation in this series was the PEP stock of the early 1970s. Some types built since have been given semi-official designations in this style.
This is different from the system used by British Rail (adopted from the LNER) to indicate the type of non-powered coach — see British Rail coach designations.
Unit numbers were allocated from 1001, following the 1-1000 set numbers of semi-fixed formations of hauled coaching stock. Different types of unit were given numbers:
Unpowered trailer units for working with 3SUB stock (this series was later expanded to include 989-1000)
3SUB (ex-LSWR units with 1200 added to their original numbers)
Two-car inner-suburban (e.g. 2NOL)
Two-car outer-suburban (e.g. 2BIL)
Four-car outer-suburban (4LAV)
Four, five and six-car express (e.g. 4COR, 5BEL and 6PUL)
Four-car inner-suburban (4SUB and 4DD)
British Railways four-car inner-suburban (4EPB)
British Railways two-car (e.g. 2HAP)
British Railways four-car outer-suburban (e.g. 4CIG)
Temporary eight-car (8VAB)
Departmental (non-revenue earning)
This series was perpetuated by the Southern Region with modifications, as older set numbers were reused for the following different types:
Parcels and departmental (Sxxx units renumbered)
Trailer Control (first digit indicated number of cars, e.g. 301 = 3TC)
Diesel Electric Multiple Units (e.g. 6S)
2PEP reused number 2001, 4REP from 3001 and 4PEP 4001/4002. This series was abandoned in 1983, when units were renumbered to fit in with the TOPS classification system, which had nominally been in use for a decade. Even then, many units displayed only the last four digits, dropping the first two digits - e.g. unit 412 301 would have the number "2301" applied. Only with later units and 2xx series DEMUs were the full numbers shown, e.g. Classes 456, 465 and 466, and some Class 455 sets, carry full 6-digit numbers.
^ abcIan Allan ABC, 1968British Railways Motive Power Survey, p.15 "2NOL, 2HAL and 2BIL ... no lavatory, half lavatory and both lavatory, depending on whether toilet facilities were provided in either one or both coaches of a two-car set or not provided at all"
^Ian Allan ABC, 1968British Railways Motive Power Survey, p.15 "The 2HAP provides similar accommodation to a 2HAL, but is fitted with electro-pneumatic brakes"
^ abIan Allan ABC, 1968British Railways Motive Power Survey, p.15 "the letters IG (4CIG, 4BIG), the old LBSCR telegraphic code for Brighton"
^The three 5BEL units were originally numbered 2051-3, but later renumbered 3051-3 to conform with the general scheme (Southern Electric by G.T. Moody, Ian Allan Ltd., various dates).