By 1938 the Southern Railway had electrified virtually all of its suburban network and had built large fleets of 3-car motor units and 2-car trailer sets to operate passenger services over these routes.  Most off-peak services were operated by single 3-car units.  Peak services were typically operated by two 3-car units flanking a 2-car trailer set, though some used two 3-car motor units without a trailer set and services to Caterham/Tattenham Corner were formed with up to three 3-car units.

Some of the Southern's 3-car motor units had been built new in 1925 and others were converted from former LBSCR a.c. electric stock but most consisted of bodywork adapted from former LSWR, LBSCR or SECR steam-hauled rolling stock, much of which dated back to the 1890s.  These bodies were cut and spliced as necessary and mounted on newly-built SR standard underframes, with new driving cabs and guard's compartments.  The policy of re-using older bodywork helped keep the cost of electrification down, avoided premature write-offs of existing locomotive-hauled suburban stock and enabled the Southern's workshops to focus on the urgent task of renewing the company's long-distance fleets.

The 4 Sub concept

By 1938 suburban electrification was paying dividends - literally.  Rush-hour traffic was up 35% and off-peak traffic was up 55% in the ten years to 1935  Electric traction had halved suburban train operating and maintenance costs.  It had always been intended that the reworked material would have a limited life and, now that financial resources and workshop capacity were becoming available, the Southern began to plan the renewal of its suburban fleet around the concept of operationaly homogeneous 4-car units - 4 Sub.

All the two-car trailer sets were to be eliminated.  Cars from those sets that had ex-LSWR bodywork on SR standard 62 ft. underframes would be inserted into 3-car motor units to make them up to four cars, formed DM-T-T-DM, designated 4 Sub.  Those trailers still on original, non-standard underframes (variously 46 to 54 ft. long) were to be withdrawn and replaced by new cars that would similarly be used to augment further 3-car units.  Moreover, over about ten years, all of the cars with pre-Grouping bodywork on standard SR 62 ft. frames would be rebuilt with new bodies, and the remaining non-standard vehicles would be withdrawn.

To compensate for the depletion in the number of units in service during the rebuilding programme, and ultimately to replace stock unsuitable for rebuilding and to provide for increased peak traffic, a certain number of entirely new 4 Sub units would be built, 30 in the first instance.

War and austerity

The new-build 4 Sub programme had barely started before Britain went to war in September 1939.  The Southern's workshop capacity was diverted to production of war materials - Eastleigh Works made parts for Blenheim aircraft, fast motor boats, landing craft, bomb trolleys, tank-carrying bogie well wagons and tail units for aerial torpedoes.  The company's remaining workshop resources were largely taken up with essential maintenance for the Southern's existing fleet, which was carrying hugely increased volumes of passenger and freight traffic.  Furthermore, some 58 cars of pre-war suburban electric stock were lost to enemy action during the war and many more written off in accidents and shunting mishaps during the blackout.

As the end of the war came in sight, the Government allowed car building to resume and it seemed possible to pick up the threads of the 1939 programme.  However there were persistent shortages of labour - specialist craftsmen in particular - and difficulties in obtaining timely deliveries of electrical equipment and other materials, many categories of which remained under Government control during the difficult post-war 'austerity' years.  Moreover, there were considerable arrears of rolling stock maintenance and repair to be taken in hand.  The Southern Railway's works managers and production planners therefore had to struggle to satisfy competing demands on limited and uncertain resources and often had to change their plans.

Augmentation, new build and rebuilding

The post-war programme had three main elements:

• Re-formation of pre-war 3-car motor units to 4 cars with suitable trailers from 2-car trailer sets and other withdrawn units.
• Augmentation of pre-war 3-car motor units to 4 cars with newly-built trailers.
• Construction of 4-car units, some all-new, others with new bodywork on frames and bogies recovered from pre-war units.

There was considerable overlap between these three lines of activity; new construction alternated between complete units and new trailers for existing units, whilst the programme of re-formation was carried out in parallel.

At the outset there seems to have been a review to choose units most worthy of augmentation.  Blocks of new unit numbers in the 4xxx  range were allocated for new build units and for the various groups of 3-car units to be augmented.  This revised plan was further adapted to circumstances as the programme was implemented.

Some 3-car motor units were taken directly into the rebuilding process without first being augmented to 4 cars.  From late 1949 onwards, once the 3-car motor units and trailer sets had been eliminated, augmented units with standard underframes were taken into the rebuilding process to emerge as 'new' 4 Sub and, later, 4 EPB units.  Where these units already had an all-steel trailer the trailer was incorporated into a 'new' unit.

Replacement 4-car units had been completed at the rate of about 50 per year during 1949-51, but during 1952-53 this rate was halved, and only 50 new units went into service.  The cause was the Government's diversion of sheet steel to an armaments programme and to the motor industry, which was pursuing an exports drive.  During 1952 especially, few entire units were withdrawn, but individual cars in poor condition were condemned and others were moved between units to cover the gaps.  This caused some units to include old cars of mixed origins: at least one ended up with cars of all three pre-1923 lineages.  Some units outside the 4131-4257 group acquired two old trailers each, and conversely a few units ended up with two steel trailers.

In summary, the 4-car augmented units occupied the ranges of unit numbers 4131-4257, 4300-4354, 4401-4594, 4601-4608, 4610, 4613, 4614.  Many of these units were short-lived and the numbers 4250, 4406, 4439, 4442 and 4483 were used twice over.

The second generation of new 4 Sub units, delivered from 1941 onwards, were numbered in the ranges 4101-4130, 4277-4299, 4355-4377, 4601-4607 (the second use of this number series) and 4621-4754.

Later re-formations in the 1950s re-used the number series 4501-4518.

Specifications

The new units and the newly-built augmentation trailers had wide bodies with curved bodysides designed for six-abreast seating.  Prior to 1946 these new cars had steel bodysides but conventional timber/canvas roofs but during that year the Southern set up a production line at its Eastligh works, constructing all-steel bodies using jigs.

Initially the new units had the same all-compartment format of the earlier stock but, after an experimental build in 1946, the standard interior layout for later builds became predominantly saloons with central corridors.  All of the units had slam doors to each seating bay.

The old and new units were similar both mechanically and electrically, could work in multiple with each other and were used interchangeably throughout the Southern electric network.

By 1938 most of the pre-war 3-car motor units were on SR standard underframes with SR standard trailing bogies.  The only exceptions were the 1925 build of 3-car motor units for the Western section (augmented as 4 Sub 4300-4325) and the majority of the conversions of former LBSCR AC stock (augmented as 4 Sub 4527-4571).  Similar SR standard underframes were used from 1939 onwards for new build 4 Sub units and for new augmentation trailers.

Prior to 1936, traction electrical equipment was supplied by Metropolitan-Vickers and used electro-magnetic control gear, mounted at the rear of the motorman's compartment.  From 1936, English Electric became the Southern's supplier and they introduced a more modern design with electro-pneumatic control gear and resistance grids mounted beneath the underframe.  The English Electric system was designed to be backwards-compatible with the earlier design so that both types could operate in multiple with no operational constraints.  No change was made to the traction equipment of augmented pre-war units but rebuilt DMBTs were fitted with new English Electric equipment.

Type 339 traction motors, supplied by Metropolitan Vickers and later by English Electric, wers standard on pre-war and early post-war units.  Following a successful experiment with unit 4130, a new design of lightweight self-ventilated motor, designated EE507 was fitted to units 4277-4299 and to many subsequent builds of new and rebuilt 4 Sub units.  However, due to shortages a substantial number of later units were fitted with the older EE339 type of motor.  Units with EE507 motors had protected air intakes on the roof and vertical ducts leading down to the motors.  It was easy to distinguish by ear between the two types of motor, by the tone and timbre of the gear noise.  The 4-Sub DMBTs with EE507 motors seemed to be more prone to tyre-flats than those with 339 motors, possibly because the leverages in the brake rigging had not been revised to take account of the lower dead weight and rotary inertia of the 507 motors.

The EPB programme

Despite the diversity of sub-types and the use of two different types of traction equipment, all of the units in the 4 Sub fleet were able to work in multiple with each other.  They were also fully compatible with the Southern electric multiple unit fleets for main line stopping trains and other local services (4 Lav., 2 Bil., 2 Hal., 2 Nol.).  This gave the Southern considerable operational flexibility and helped minimise staff training costs, spares holdings, etc.

However, by the late 1940s, the technology was becoming out of date.  The Westinghouse brake, though reliable and fail-safe, was not instantaneous in operation - the reduction in train pipe pressure following a brake application took a few seconds to propagate to the rear coaches of a long train.  Shunters had to climb down on to the track between units to couple and uncouple units.  Motor-generator sets offered a better source of low-voltage power for control circuits than potentiometers and were already used in the Southern's 'express' EMU fleets.

For all of these reasons, by 1950 it was time for the Southern to make a step-change to a new standard EMU type.  The first units to be built to the new standard were known as 4 EPB (Electro-Pneumatic Brake).  They were very similar to the 4 Sub.s, were built on the same jigs and many incorporated former Sub augmentation trailers.  Electrically and mechanically, however, they were very different and were therefore unabl to work in multiple with any of the earlier types.  As well as the instant-acting EP brake (with the Westinghouse brake as a fail-safe backup) the EPBs had semi-automatic 'Buckeye' couplers in place of the Sub's screw-type couplers.  Brake hoses, as well as control jumpers, were mounted on the cab front, enabling coupling and uncoupling operations to be performed from the side and at platform level.

The 4 EPBs were the first in a series of types that were built from the early 1950s to the early 1970s, all of which could operate in multiple with each other (with some restrictions on formations) and this interoperability was later extended to include some diesel and electric locomotive types.

Deployment

The 4 Sub fleet was deployed across the entire Southern suburban network.  During the 1950s, they were displaced from some suburban routes, notably (though not exclusively) from the South Eastern Division, where ten-car trains were introduced using EPB stock.  Many of the replacement EPB vehicles included recycled components from withdrawn 4 Sub units.

Because 4 Sub units of any age were compatible and could work in multiple with each other, 8-car peak formations could be any combination of pre- and post-war units, with Metrovick or English Electric equipment.  As well at the quotidian task of operating suburban routes, 4 Sub units had occasional outings to other destinations.  Being compatible with the fleets used on stopping and semi-fast services on the main lines, they were sometimes used to strengthen longer distance services in multiple with 4 Lav or 2 Bil/2Hal units.  They were also frequently used on busy summer weekends for trips to Brighton and other seaside destinations or for special services to race meetings, the Farnborough Air Show and other events.

Withdrawal

By the end of 1956 almost all of the augmented pre-war 4 Sub units with SR standard underframes had been withdrawn for rebuilding, either as 'new' 4 Sub or as 4 EPB units.  Of the pre-war fleets there remained the augmented 1925-built units 4300-4354 and very few others except units in the range 4527-4590, almost all of whose old cars came from 3-car units 1717-1772 and had non-standard frames with components from a.c. cars.  Most of these latter were condemned in 1956 but 72 cars were retained and reformed into 18 4-car units numbered 4501-4518 (the second use of these numbers), to allow for the withdrawal and eventual replacement of the Windsor 2 Nol. units.  Two of these 18 units were withdrawn in 1958, most in late 1959, and the last three, comprising the last Southern Electric cars with pre-grouping bodywork in passenger service, in January 1960.

Units 4300-4354 (augmentations of the 1925-built units 1285-1310 and 1496-1524) were the next to go, withdrawn during 1960-61.  A few of the second batch survived officially into January 1962, though they were probably already out of service by the turn of the year.  Their effective replacements were BR-designed 4-EPB units 5301-5356.

Faced with a drop in traffic levels in the early 1970s, BR started to withdraw some of the post-1939 4 Sub units, focusing on all-compartment units and those with the older English Electric 339 motors.

All of the 'type 339' units were withdrawn by the mid 1970s, including some with saloon accommodation.  In view of public concerns about personal security in non-corridor stock and high levels of vandalism of compartments, the opportunity was taken to swap saloon trailers from withdrawn units for compartment trailers in units remaining in service.

The remaining 4 Sub units were withdrawn in the early 1980s and the last passenger service to be operated by 4 Sub units (10.21 East Croydon-Victoria, 4279 and 4754) was on Tuesday 6^th September 1983.

Summary timeline