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Southern Railway 4-SUB British Rail Class 405
4101 (prototype) in 1941
In service	1941-1983
Manufacturer	Eastleigh Works
Constructed	1941-1951
Number built	185 units
Number preserved	1 unit
Formation	DMBSO-TS-TSO-DMBSO
Capacity	368 - 386 Seats
Operators	Southern Railway British Railways
Depots	Wimbledon Traincare Depot (London)
Specifications
Car length	DMBSO: 62 ft 6 in (19.05 m) TS & TSO: 62 ft (18.90 m)
Width	9 ft (2.74 m)
Maximum speed	75 mph (121 km/h)
Weight	134 long tons (136 t; 150 short tons)
Traction motors	Four
Power output	Early units: 4 x 275 hp (205 kW) total 1,100 hp (820 kW) Later units: 4 x 250 hp (190 kW) total 1,000 hp (750 kW)
Current collector(s)	Contact shoe
Braking system(s)	Air (Westinghouse)

The Southern Railway (SR) and the British Railways (Southern Region) (BR(S)) used the designation Sub to cover a wide variety of electric multiple-unit passenger trains 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.

In 1923, the Southern Railway inherited 84 3-car suburban electric multiple units from the London and South Western Railway (LSWR), dating from 1914.^[1] These could be enhanced by 24 2-car trailer sets coupled between two units, creating 8-car formations. Over the next 18 years the SR acquired a further 516 3-car sets.^[2] These were either new; rebuilt from steam stock inherited from the LSWR, the London Brighton and South Coast Railway (LBSCR) and the South Eastern and Chatham Railway; or rebuilt from LBSCR AC multiple units. Some rebuilt units had ‘torpedo style’ front ends, others had the newer, flatter, front ends used on the main-line stock introduced during the early 1930s. They were designated ‘3-Car Suburban Sets’ to differentiate them from main-line units, and were numbered between 1201 and 1800. They are sometimes referred to as ‘3-Sub units’, but this designation does not appear to have been used whilst they were in use.

The 3-car units ultimately proved to be inadequate for most trains and inconvenient as the trailer sets had no driving cabs and so had to be marshaled between two 3-car units. Therefore, many were augmented by the addition of a new trailer car and rebuilt as 4-Sub units between 1942 and 1948. The remainder of the 3-car units were withdrawn. The last 2-car trailer set was withdrawn in September 1948, and the last 3-car formation ran in 1949.^[3]

In 1939, following the appointment of Oliver Bulleid as Chief Mechanical Engineer, the SR announced a new welded steel 4-car suburban unit with curved sides, seating six passengers across rather than the normal five.^[1] The prototype unit, 4101, was not completed until 1941 due to the advent of the Second World War. As built, this had composite (both First Class and Third Class) seating accommodation in one trailer car and domed cab roofs, but because first-class accommodation in the London suburban area was withdrawn in October 1941, the six first-class compartments were declassified to third-class before the unit was placed in service.^[4] A further nine units were built in 1944-5 with Third Class accommodation only. Twenty units were introduced from 1946 with a vertical, ‘slab’ front end, and Third Class accommodation only. The underframes were built at Lancing Carriage Works and the bodies at Eastleigh Works.

As mentioned above, between 1942 and 1948 many 3-Car Suburban Sets were rebuilt and enhanced by the addition of a new 'Augmentation' trailer car to create 4-Sub units. This was done by inserting new steel trailers rather than one of the existing trailer cars. The rebuilt units were renumbered between 4131 and 4249.

BR(S) continued to introduce 4-Sub units, both new and converted from existing units, until 1951.

The usual formation for the newly built units was to have a Driving Motor Brake Third (DMBT) car at each ends, with accommodation for 108 passengers in nine compartments, together with two Trailer Third (TT) cars between with ten or eleven compartments giving a total passenger accommodation for 456 in all four coaches. After the renaming of 'Third Class' to 'Second Class' in 1956 these became DMBS and TS respectively.) However some of the rebuilds had Trailer Composite (TC) cars. The earlier coaches provided compartments, whereas the post war newly built carriages tended to provide saloon or semi-saloon style accommodation to allow for more standing room. These new 'open' carriages were designated DMBTO or TTO.

The 3-car suburban units had included a proportion of first-class accommodation, usually seven first-class compartments providing 56 seats together with 18 third-class compartments seating 180. Normally, all of the first-class compartments would be in the central car, this being a trailer composite, although some units had two or three of the seven first-class compartments in one of the end cars, this being a motor composite brake. The new 4 SUB units were also designed to include first-class accommodation, the first ten units (nos. 4101–10) being built with two nine-compartment motor third brakes, one eleven-compartment trailer third, and one ten-compartment trailer composite having six first-class compartments in the middle, flanked by two third-class at each end. First-class compartments seated ten each, third-class compartments twelve, for a capacity of 60 first- and 396 third-class. Before unit 4101 could enter service, first-class was suspended for the duration of the war (never being reinstated); the first-class compartments of no. 4101 were redesignated third-class, and the unit now provided 468 third-class seats. The remaining nine units of the first batch, nos. 4102–10, which were not built until 1944–45 were entirely third-class from the outset, with seating capacity similar to no. 4101. Units built subsequently had one fewer compartment in each car, giving a total capacity of 420 third-class seats in units 4111–20.^[5]

Throughout their careers the 4-Sub units were used on the London suburban services of all three divisions of BR (S).

179 out of the 184 new units constructed between 1946 and 1951 were still surviving at the introduction of the TOPS computer system in the early 1970s but were expected to have limited working lives. Under TOPS all surviving units were allocated to Class 405, but divided into two sub-classes. Sub-class 405/1 included the former Southern Railway sets numbered between 4112 and 4387, and sub-class 405/2 included the former British Railways sets built after 1949 and numbered between 4601 and 4754. All had been withdrawn by 1983.

As withdrawals reduced the number of units, services were concentrated in certain areas so that by 1981 units working on the Central section were allocated to Selhurst Depot and were concentrated on London Bridge to East Croydon services, and on peak-time commuter services to/from Holborn Viaduct and Blackfriars. Units on the South Western section were allocated to Wimbledon Depot and were concentrated on services between Waterloo and Effingham Junction, Shepperton or Chessington, although all of the scheduled services were on weekdays.^[6][7]

The rebuilt units began to be withdrawn in the late 1950s and all had been withdrawn by the end of the 1960s. Apart from a few accident damaged units, the newly built units began to be withdrawn in 1972, and had all been withdrawn by 1983.^[8] Those units surviving in the early 1970s were designated as Class 405 under the TOPS computer system.

In 1972 a unit was converted for use transporting stores between Eastleigh Works and various Southern Region depots. Renumbered 024 the unit was based at Slade Green and followed a set route on a weekly schedule serving Stewarts Lane, Ashford, Hither Green, Slade Green and Selhurst.^[9][10][11]

In 1974 unit 4367 was retired from revenue-earning service and converted for use as a mobile instruction train and was allocated to Ashford Depot. As part of the conversion the unit was renumbered 055.^[12][11]

After withdrawal, some individual steel-bodied trailer vehicles were adapted for use in de-icing the third rail and were fitted with spraying and scraping equipment. These vehicles were renumbered ADS70050/1 and ADS70086/7 and were used in conjunction with stock having Electro-Pneumatic controls.^[13] In 1977 some additional conversions for de-icing were made when 6 new 2-car units were created from 4-SUB Motor Brake vehicles. These were renumbered 004 - 009.^[14]

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 was stored at the Electric Railway Museum, Warwickshire, after spending many years stored in the open at various locations, but has subsequently been moved to the One:One Collection site in Margate, Kent.^[15][16]
• Former Departmental Converted 4 Sub Carriage TT 10400 was stored at a private site, but was scrapped around 02/2020.

• The ABC of Southern Electrics. Ian Allan. 1968.
• Brown, David (2009). Southern Electric - A New History: Volume One - Development of the London Suburban Network and its Trains. Capital Transport. ISBN 978-1-85414-330-3.
• Marsden, Colin J. (1983). Southern Electric Multiple Units; 1898 - 1948. Ian Allan. ISBN 0-7110-1253-9.
• Moody, G.T (1968). Southern Electric 1909–1968. Ian Allan.
• Rayner, Bryan; Brown, David (1983). The '4 Sub' Story. Purley: Southern Electric Group. ISBN 0-906988-09-8.
• Rayner, B W; Hogie, J D (1981). Southern Region unusual train services 1981-82. Purley: Southern Electric Group.
• Williams, Alan; Percival, David (1977). British Railways Locomotives and Multiple-Units including Preserved Locomotives 1977. London: Ian Allan. ISBN 0-7110-0751-9.