Southern Railway multiple unit numbering and classification
The Southern Railway created classification and numbering systems for its large fleet of electric [multiple unit]s, 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.
Classification
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 prefixed by the number of cars in each unit, e.g. 4SUB for a four-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 to indicate the type of non-powered coach — see British Rail coach type codes.''
Unit numbering
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:| 1001–1200 | Unpowered trailer units for working with 3SUB stock |
| 1201–1800 | 3SUB |
| 1801–1999 | Two-car inner-suburban |
| 2001–2899 | Two-car outer-suburban |
| 2901–2999 | Four-car outer-suburban |
| 3001–3999 | Four, five and six-car express |
| 4001–4999 | Four-car inner-suburban |
| 5001–5599 | British Railways four-car inner-suburban |
| 5601–6999 | British Railways two-car |
| 7001–7999 | British Railways four-car outer-suburban |
| 8001 | Temporary eight-car |
| S1–S101 | Departmental |
This series was perpetuated by the Southern Region with modifications, as older set numbers were reused for the following different types:
| 001–099 | Parcels and departmental |
| 301–999 | Trailer control |
| 1001–1499 | Diesel electric multiple units |
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, 458/5, 465 and 466, and some Class 455 sets, carry full six-digit numbers.