3SUB
The 3SUB or three coach suburban unit dates from 1925 when the Southern Railway introduced a fleet of 55 sets on the Western and Eastern Sections. Of the original order, there were two distinct batches; the first 26, numbered 1285-1310, were mounted on short frames, whereas those built for the Eastern Section were mounted on standard length frames and numbered 1496-1524. On Western Section sets seating was provided for 48 first and 170 third class passengers, whereas Eastern Section sets, being slightly longer, accommodated 56 first and 180 third class passengers. Vehicles were constructed by Metropolitan Vickers Ltd.
These three car units proved highly successful, and further sets, numbered 1401-95 and 1525-34, were introduced in 1926 for use on the Eastern Section. These batches were not 'new builds' but converts from former SECR steam stock, conversion to electric taking place at Ashford Works.
During the 1927-34 period several further batches of 3SUB units emerged, the majority being conversions from steam-hauled vehicles, but as these came from a number of sources, several detail differences existed.
After some years the Southern Railway decided that fixed four coach trains would be more beneficial to operate, and during the 1940s, a start was made to alter three car units to four. This was done by adding another trailer vehicle. Some were steam stock rebuilds, previously in use as part of trailer sets, whilst others were of new construction. The last 3SUB stock operated during 1949, mainly on the South Western Hounslow Loop line where platform lengths were unable to accommodate more than six car formations. Of the original 3SUB units No. 1782 is worthy of mention, as it remained in service for many years as a CM&EE instruction unit carrying the number S10, which was later changed to 053.

TRAILER SETS
During the early 1920s passenger returns on the South Western electrified lines out of Waterloo required a revision of the service, with either additional trains being supplied or additional coaches added to existing services. The latter option was accepted, and a fleet of 24 two-coach unpowered trailer sets was introduced, being converted from steam hauled stock at Eastleigh Works. The trailer sets did not have driving positions, but on the outer ends air connections and electrical jumper cables were provided at a compatible height to the suburban electric multiple unit fleet. After the initial 24 sets were in service, further batches were introduced, all being converted from steam-hauled vehicles, and they were used on all three SR sections.
During the mid-1930s a considerable reorganisation of trailer sets took place, most being reformed to give better passenger loading, with some of the older vehicles being withdrawn. In the closing years of the 1930s other trailer sets were withdrawn, some going for scrap, while others were again rebuilt and used as additional trailers to augment 3SUB units to four coach sets.

4SUB
By far the most numerous of the SR-designed multiple units were the 4SUB (four car SUBurban) units, first introduced in the early 1940s, when set No. 4101 was outshopped from Eastleigh Works. The set was formed of two driving motor thirds with two trailers between. The decision to introduce fixed four car formations was to remove the need to use trailer sets. At the same time as the first 4SUB emerged, it was announced that alterations to existing suburban stock would take place, resulting in the reformation of 3SUB units to form four car sets, the additional trailer, in most cases, coming from disbanded trailer sets.
Unit No. 4101 entered traffic in late 1941 on the Victoria to Orpington line. The seating layout was for 60 first and 396 third class passengers, the first class accommodation being provided in a trailer composite. However, it is unlikely that this accommodation was ever actually used for first class passengers, as it was dispensed with on suburban services from October 6 1941. Over three years elapsed before the second 4SUB unit emerged from Eastleigh as No. 4102, in 1944. In 1945 production sets started to enter service with Nos. 4103-4110. By this time a vast number of 3SUB units were already in service, reformed as four car units, and thus renumbered in the 4XXX series. In 1946 Eastleigh constructed sets Nos. 4111-20, and these differed from previous 'new' SUB units by having wide compartment corridors in their motor cars. This was to accommodate the large numbers of standing passengers travelling in leading vehicles at peak periods. A further batch of identical units entered service in 1947/8 carrying numbers 4364-77. In 1946 sets Nos. 4121-30 entered service and again these were of a slightly revised layout, with coaches of a semi-open layout (three or four compartments grouped together with a gangway). After travelling in semi-open vehicles, passengers much preferred this internal design and pressure was brought upon the railway authorities to base future stock on this layout. In 1948/9, when sets Nos. 4277-99 entered traffic, these requirements had been honoured, with three of the four cars in an open design and this pattern continued on all subsequent builds. Nos. 4621-54 emerged in 1949 with Nos. 4655-4709 following in 1950, and Nos. 4710-4754 during 1951. After this, 'new' generation suburban stock, classified EPB, was introduced. Sets Nos. 4694-99, and 4718 were, when built, fitted with roller blind headcodes in place of the stencil type hitherto used, serving as a prototype for future units. A number of the 3SUB/4SUB converts were phased out as 1949/51 builds entered service, and the majority had been withdrawn by the mid-1950s. A number of SUB car reformations took place over the years, resulting in several 'odd' sets. The SUB fleet remained largely intact until the first 'new' generation Class 508 electric multiple units were delivered from 1979, and subsequently Class 455 stock from 1982/3. The final SUB units remained in regular service until autumn 1983. When built, all units were painted in green livery, and those surviving after the corporate identity colours were introduced were painted in rail blue. Prior to the final demise of the SUB fleet, set No. 4732 was repainted by Selhurst Depot in SR green livery and sunshine lettering.