Before 1948 the use of multiple units in Britain was limited. The Southern Railway had embarked on a large electrification programme in the mid-1920s, and this saw the construction of many EMUs. However, elsewhere in the country their use was restricted to a few isolated networks, mainly in metropolitan areas (Newcastle, Manchester, Liverpool and north-west London). Similarly with DMUs, most companies had tried prototypes but only the GWR went further, building a fleet of 38 streamlined railcars between 1933 and 1942. As such, away from the Southern there was little need to classify multiple units.

Electrification on the Southern actually started before the grouping of 1923. The London and South Western Railway used third-rail electrification on routes from Waterloo to Shepperton, Hampton Court, Hounslow and Guildford in 1913-1915. Rolling stock was 84 3-car units rebuilt from loco-hauled coaches and numbered E1 to E84. It appears that these were not given any form of classification at the time. A different approach was taken by the London, Brighton and South Coast Railway, which used AC overhead wiring, initially on the South London line which ran in a loop from Victoria to London Bridge via Brixton and Peckham in 1908. This was followed by the routes to Crystal Palace in 1911 and to Coulsdon and Wallington in 1925. The trains built for these were referred to as SL stock, CP stock and CW stock respectively.

Following the grouping of 1923, the Southern Railway decided to adopt the LSWR third-rail scheme as standard, and the AC overhead lines were de-wired in 1926. Further routes were electrified and new trains of various types were introduced, meaning that a method of classification became more necessary. The chosen approach was to use codes where the first numerical digit indicated the number of coaches in each unit, and the following 2, 3 or 4 letters showed the unit's configuration or intended purpose. Early examples were numerous 3-SUB and 4-SUB units for suburban duties, and 2-SL and 2-WIM units for specific routes (South London lines and Wimbledon to West Croydon respectively). Something of a fixation with toilets began with the introduction of 4-LAV units for use on the Brighton line and elsewhere from 1931, followed by the 2-NOL units with no lavatories, 2-BILs with 2 (Bi-Lavatory) and 2-HALs with half a lavatory. No, sorry, half the number of lavatories that a 2-BIL had! Further main-line trains were the 5-BEL Pullman units for use on the Brighton Belle service, 6-PULs (which included a single Pullman car), 6-CITYs (also for London to Brighton) and 6-PAN (with a pantry). The first fully-gangwayed units appeared in 1937 and were referred to as 4-COR (for corridor), these being accompanied by 4-RES and 4-BUF units for those with restaurant or buffet cars.

As will be seen, this scheme was continued by BR's Southern Region after 1948 and indeed it remained in use even after the introduction of TOPS in the 1970s.

The newly formed British Railways initially continued with the policies of its precursor companies. The Southern Region was to continue with large-scale EMU use, while new electrification schemes initiated by the LNER (on lines from Liverpool Street and between Manchester and Glossop) were introduced by the Eastern Region. The economies to be gained from multiple-unit operation were recognised but it was not until the mid-1950s that their use really took off.

After several small batches of DMUs and railcars were built from 1953 onwards, the use of DMUs with (mainly) mechanical transmission was selected as the way forward and about 1,000 units were constructed over the space of the few years from 1957 to 1963. The quantity is a bit vague because the units were not operated in fixed formations. Just under 3,500 individual coaches were built, by a wide range of firms, and these were formed into (mainly) 2-, 3- or 4-car units. There were also single units, and some express units were of 6-car length (e.g. Trans-Pennine units). As the formations were not fixed, there was no attempt to give complete units their own numbers (although some regions did apply set numbers, notably the Western and later the Scottish regions). There was also no standard form of classification. Units were referred to be a combination of their maker and their configuration, as in Derby 4-car Suburban, Swindon 3-car Inter-City and so on.

On the Southern Region, most lines were already equipped with third-rail power, but there were still a number of routes where electrification could not be justified, either on grounds of cost or other factors. Diesel units were therefore required but the Southern decided not to use the format that had been standardised on elsewhere on BR (underfloor engines and mechanical transmission). The preference was instead for units with engines inside the bodywork and electric transmission. A range of types was built between 1957 and 1962, these being operated in fixed-formation, numbered in the 4-digit Southern series and given classifications based on the SR EMU scheme. First up were narrow-bodied units for the Hastings line, these all being 6-car units and classified as 6-S (for those with short (58ft) bodies), 6-L (for similar units but with 64ft bodies) and 6-B (for units with buffet cars, which were later removed to result in 5-L units). These were followed by 2-H and 3-H units (for use in Hampshire) and finally the 3-D units. The latter were intended for use in East Sussex and the use of the letter D seems to have been taken from the end of 'Oxted' (REF x, Southern Region Multiple Unit Trains 3rd edition, Beecroft and Rayner, Southern Electric Group, 1984) since a 3-O could potentially be confusing. Reformations of various units also saw types 3-R and 3-T added, the former being for Reading to Redhill, while the latter referred to the addition of an EPB trailer car into the 2-H units.

While looking at Diesel-Electric Multiple Units, mention needs to be made of the Blue-Pullman units. Built in 1959 for use on the Midland and Western regions, these were similar to the Southern Region DEMUs in that they housed the engine within the bodywork. Unlike the slab-fronted SR units, the Blue-Pullmans had streamlined cabs and, as their name suggests, the passenger accommodation was to a much higher standard of luxury. As with BR's mechanical DMUs, the Blue Pullmans had not set numbers, and no specific classifications until the introduction of TOPS (q.v.).

BR's Total Operations Processing System (TOPS) involved the computerisation of records concerning rolling stock, and this required some standardisation in terms of stock numbering and classification. All multiple units were to be given class codes in a series of blocks determined by their power source.

1xx class numbers were for Diesel (Mechanical or Hydraulic) Multiple Units
2xx class numbers were for Diesel Electric Multiple Units
3xx class numbers were for AC overhead Electric Multiple Units
4xx class numbers were for Southern Region DC 3rd Rail Electric Multiple Units
5xx class numbers were for all other DC Electric Multiple Units (which included both overhead and 3rd rail types)

The initial allocation of TOPS class codes to D(M)MUs assigned a separate code not only to each make of unit but also to different types of coaches within the units. Driving Motor coaches were classified in the range 100 to 129, Driving Trailer coaches between 140 and 148 and Trailer coaches from 160 to 189. I have been trying to discern the logic behind the allocation of individual class codes but there does not seem to be much, with no regard being made to the age of the units, their make or their intended area or type of operation. The only patterns that I could find were that Driving Motor coaches on short-frames (i.e. 57ft-long) were all in the 100-113 range (with the higher numbers applied to units with Rolls-Royce engines), while DMs with long-frames (64ft) were 114-127. Classes 128 and 129 were for non-passenger (parcels) units, later joined by conversions from classes 116 and 122 which became 130 and 131 respectively. The single-car passenger units became classes 121 and 122, sitting in a range otherwise occupied by cross-country and Inter-City units. If anyone can comment on the significance of the chosen class numbers I would be interested to hear from them.

The Southern Region DEMUs were assigned class codes in the 201-207 block, with the Hastings express units becoming 201 (6-S), 202 (6-L) and 203 (6-B), the Hampshire 3-H units becoming 205 and the Oxted 3-D units 207. Codes 204 and 206 were applied to the re-formed 3-T and 3-R unit respectively. The only other DEMUs were the Blue Pullman units. These were assigned class codes 251 (for the Driving Motor cars) and 261 (for the Non-Driving Motor and Trailer cars), reflecting the policy for non-fixed multiple units as had been applied to other DMUs.

The assignment of new TOPS class codes to EMUs on the Eastern and Midland regions was fairly logical. For AC units, the TOPS class codes were based on the pre-existing AMx designations. AM2 units became class 302, AM3 became 303 and so on. Class 301 did not appear as the AM1 units converted from DC to AC for use on the Lancaster to Morecambe and Heysham lines in 1952 had already been withdrawn. DC units did not have any existing classifications, and the TOPS class codes assigned appear to have been based on the line voltage used. Hence the units used on the Watford and North London lines ran on 630 volt 3rd rail and became class 501. The Merseyside units took 630/650 volt (also from a 3rd rail) and became classes 502 and 503. The Bury units used a unique 1,200 volt side-contact system and became class 504. Finally the Manchester to Altrincham and Manchester to Glossop/Hadfield units were both fed with 1,500 volts from overhead wiring and were allocated classes 505 and 506, although in the event the former were withdrawn before TOPS was implemented.

A distinctly logical approach was taken to the allocation of TOPS class codes to Southern Region DC EMUs, although it was to be extensively revised within a few years. Technical advances had seen a succession of improvements made to the electrical equipment and control systems of SR EMUs, and because these had some operational implications (in terms of compatibility), there was a system of referring to units by the 'generation' of their equipment. Pre-BR units were referred to as 1936 stock, while BR-built units were either 1951 stock, 1957 stock, 1963 stock or 1966 stock. The first iteration of TOPS numbering used these as a the basis for the second digit of the class numbers, with the third digit simply being sequential. As such, the remaining pre-BR units became class 401, 402, 403, 404 and 405. Units of 1951 stock became classes 411 (4-EPB), 412 (2-HAP), 413 (2-EPB) and 414 (prototype 4-BEP and 4-CEP). 1957 stock became 421 (production 4-BEP and 4-CEP), 422 (MLV and TLV), 423 (4-EPB) and 424 (2-HAP). 1963 stock was 431 (4-BIG and 4-CIG), 432 (4-VEP), while the 1966 stock became classes 441 (4-REP) and 442 (4-TC). Non-standard stock was assigned class codes in the 45x range and comprised 451 and 452 (Isle of Wight 3-TIS and 4-VEC respectively) and 453 (Waterloo and City line stock). The recently-built PEP units of prototype suburban stock started a new generation with class codes in the 46x range. For these trains a different approach was proposed, in that class codes would refer to the different types of coach within a unit. It was intended that driving motor cars would be class 461 and non-driving motor cars would be 462 (there were no trailer cars). It would appear that this was later revised so that class 461 referred to the 4-car units and class 462 to the 2-car unit, although this is not confirmed and in any case entirely new codes were soon to be allocated.

Sub-classes were used to distinguish variants, even though some of the differences were quite major. For example class 414/1 was for the prototype 4-CEP units and 414/2 was for the prototype 4-BEPs (with buffet cars). The production batch were similarly 421/1 for 4-CEPs and 421/2 for 4-BEPs, with a similar distinction for class 431 4-CIGs and 4-BIGs. The situation with the luggage vans was also slightly odd. The Motor Luggage Vans (with 3rd rail electric motors and battery back-up) were classified as 422/1 while the Trailer Luggage Vans (which were essentially just gangwayed brake coaches modified with through wiring and brake pipes) were class 422/2. All of this was somewhat at odds with the way TOPS was implemented for other types, whereby major differences were distinguished by separate class codes while more minor differences were covered by sub-classes. This was to be addressed in a revision to the Southern TOPS classes a few years later (q.v.).

Changes were made to the TOPS class codes assigned to D(M)MUs in about 1979. The main change was that the use of separate class codes for different types of coaches was abandoned. So, for example, the BRCW class 104 Driving Motors had previously operated with class 140 Driving Trailers, class 160 Trailer Seconds, class 166 Trailer Brakes and class 169 Trailer Composites. After 1979, all of these coaches were referred to simply as class 104. The second change was that the distinction between Leyland-engined units and AEC-engined ones was no longer signified by a separate code. Hence Leyland-engined Metro-Cammell units were changed from class 102 to class 101, and likewise Leyland Cravens units of class 106 were integrated with AEC Cravens in class 105. Rolls-Royce engined units were not affected, with RR Metro-Cammell units remaining as 111.

As already shown, the initial allocation of TOPS class codes to Southern Region units was logical but had a few anomalies. For example, classes 413 and 424 were applied to 2-HAP units that differed only in their control equipment, while conversely class 414 was allocated to units both with and without a buffet car (4-BEP and 4-CEP). The whole system was revised in about 1975 with most types getting new codes. The second digit was still based on the equipment generation, although amended such that the 1951 and 1957 stock were combined as classes in the 41x block. 1963, 1966 and 1972 stock all thus moved down to the 42x, 43x and 44x blocks respectively. Tube stock was moved up from 45x to 48x, while un-powered units were now to be in 49x. Another major change was that the third digit was no longer sequential but was dictated by the configuration of the unit. 4-car express units with buffet cars would have a 0 as the third digit, those without buffets would have 1. Class codes ending in 2 were not initially used, semi-fast units ending in 3 (for 4-car units) or 4 (2-car units). Similarly, suburban units would end in 5 (4-car units) or 6 (2-car units). Parcels units would have codes ending in 9. Digits 7 and 8 were not officially allocated but were used for Waterloo and City line stock (487) and for de-rated semi-fast 2-car units (418). The old and new codes are shown in the table below. Table to follow

By the early 1980s, BR was considering how to replace the large number of first-generation DMUs, which were by then around 20 years old. The initial plan was to use coupled 4-wheeled railbuses for lightly-used routes and bogie units with overfloor engines and electric transmission for more heavily-used ones. Prototypes of each were built, these being designated as class 140 and 210 respectively. The class code of 140 re-used a code that had previously been in use until 1979 for DMU driving trailers, while the code of 210 started a new block after the existing Southern Region DEMUs (classes 201 to 207). The railbus idea was pursued and the prototype was followed by production batches of classes 141, 142, 143 and 144, collectively known as 'Pacers'. However the 210 design proved to be over-engineered and too expensive. An alternative was to build further prototypes, this time with underfloor engines and hydraulic transmission. These were class 150 (built by BREL) and 151 (by Metro-Cammell) and the former was successful, leading to production batches of classes 150/1 and 150/2, dubbed 'Sprinters'. Further Sprinters were built later in the 1980s, these being intended for longer distance work. The coaches were longer (23m instead of 20m) and had passenger doors at each end rather than at the 1/3 and 2/3 positions used for the suburban Sprinters. These 'Super Sprinters' were built by Leyland Bus (class 155) or Metro-Cammell (class 156), and it is presumed that a gap was left in the series in case further suburban units were built. In the event none were, though two of the missing numbers were eventually used, for single-car DMUs converted from class 155 units (class 153) and a prototype class 150 unit with modified equipment and interiors for the forthcoming Express Sprinters (class 154).

AC EMUs new in the 1980s and early-1990s were classified in quite a structured way. Aluminium-bodied units of classes 313, 314 and 315 were built, based on the PEP prototype units. Class 316 was intended for use on a new underground link across central Manchester which was cancelled. Class 317 was the first of a new generation of unit using steel bodies based on the Mk3 coach construction. It was followed by similar classes 318, 319, 320, 321 and 322.