A number of different numbering and classification schemes has been used for locomotives and multiple units on Britain's railways, and this page explains the principal systems that have been used. Locomotives and multiple units (the majority being self-propelled) have frequently had similar arrangements for classification and numbering, so are considered together here. There are also links to other pages that deal in greater depth with the particulars of individual types.

Pre-Nationalisation

Before Nationalisation in 1948, each railway company had its own numbering and classification arrangements. Some of the principal examples are outlined below.

Great Western Railway (GWR)

The GWR was the longest-lived of the pre-nationalisation companies, surviving almost unchanged the 'Grouping' of the railways in 1923. As a result, the history of its numbering and classification of locomotives is more complicated. The original Bristol Temple Meads station, first terminus of the GWR, is the building to the left of this picture The Great Western Railway (GWR) was a British railway company, linking South West England, the West Country and South Wales with London. ...

Numbering

Broad-gauge locomotives were not numbered, only named, with the exception of those that were absorbed from other railways, which took numbers in the 2000-2199 series.

Initially standard-gauge locomotive numbering was a simple sequential system, starting from 1. However, on occasions new locomotives would re-use gaps left by the withdrawal of older classes, especially where they were designed as direct replacements. Later, express passenger locomotives were numbered in a consecutive series commencing at 3000.

Subsequently, a system was adopted for new locomotives where the second digit indicated the broad type of locomotive. For example, express passenger locomotives had x0xx numbers and large mixed traffic tender locomotives were x9xx. When a class numbered more than 100 locos, rather than continue the numbers consecutively the second digit remained constant (e.g. 4900 Class included 4900-4999, 5900-5999, and 6900 onwards). From this time on, numbers below 2000 were mainly occupied by old, absorbed or otherwise non-standard locomotives, including the experimental diesel locomotives used by the GWR.

In 1923, when the locomotives of a number of small railway companies were inherited, these were renumbered into gaps in the original 1-1999 series, broadly according to their wheel arrangement. Many of these were withdrawn after a short period of time, but those that survived in 1946 were subject to another renumbering to rationalise the system further.

Classification

A very simple system was adopted, whereby the name (for broad-gauge locomotives) or number of the first locomotive in a class became the classification for all locomotives in that class (e.g. 'Sun Class', '4000 Class'). After the end of the broad gauge, names were applied to principal passenger and mixed-traffic standard-gauge locomotives. These were often based on a single theme, which could also lend its name to describe a class, for example 'Stars', also known as the '4000 Class', whose names included 'North Star', 'Rising Star' etc.

Southern Railway (SR)

The SR made extensive use of electric multiple units, extending the former London & South Western Railway 660 V DC third rail network across large swathes of its operating area, and so built very few steam locomotives of its own. The Southern Railway in the United Kingdom was the smallest of the four railway systems created in the Grouping ordered by the Railways Act 1921. ...

Locomotives

Perhaps because the focus of the SR was on electrification, no standard classification of steam locomotives was developed. Instead, the classes allocated by its pre-Grouping constituents were perpetuated. Initially, the numbers of the inherited locomotives were prefixed by a letter derived from the first letter of the main locomotive works on each section to avoid duplications, but later the letter was dropped and numbers were increased, as follows:

• Ex-South Eastern & Chatham Railway: 'A' (for Ashford), then 1000 added
• Ex-London Brighton & South Coast Railway: 'B' (for Brighton), then 2000 added
• Ex-London & South Western Railway: 'E' (for Eastleigh), then numbers left unchanged

Old LSWR locomotives had been renumbered into a duplicate list by adding '0' to the front of their original number (e.g. 0298). When the prefix letters were dropped, they had the '0' replaced with a '3' (e.g. 0298 became 3298).

In the 1930s the SR adopted a numbering system for new-build locomotives based on their wheel arrangement according to the UIC classification scheme, eg, C1-40 (for the SR Class Q1 of forty 0-6-0 locomotives), etc. The UIC classification is a comprehensive system for describing the wheel arrangements of locomotives. ... Jump to: navigation, search Preserved number C1 33038 at Bricklayers Arms in London during BR era. ...

Multiple Units

For a full explanation of the numbering and classification arrangements of SR electric multiple units, see SR Multiple Unit Numbering and Classification. The Southern Railway created numbering and classification systems for its large fleet of Electric multiple units that were perpetuated by the Southern Region of British Rail until the early 1980s, when the impact of TOPS was felt. ...

In brief, multiple units were classified by giving each type a three-letter code (sometimes two-letter) prefixed by the number of carriages within each set, e.g. 4-Sub for a 4-car suburban type set. Unit numbers were arranged in batches of similar types from 1001 upwards, following the series for set formations of hauled coaching stock, which were numbered 1-1000. These arrangements were perpetuated by the Southern Region of British Rail until the early 1980s, when the impact of TOPS was felt. Logo of British Rail British Railways (BR), later rebranded as British Rail, ran the British railway system from the nationalisation of the Big Four British railway companies in 1948 until its privatisation in stages between 1994 and 1997. ... This article is about the railroad stock management system TOPS. For the DEC PDP-10 operating systems, see TOPS-10 and TOPS-20 The Total Operations Processing System, better known by its initials TOPS, is a computer system for managing the locomotives and rolling stock (railroad cars) owned by a...

London Midland & Scottish Railway (LMS)

The LMS was principally reliant on steam locomotives, though it was the leader in testing new diesel locomotives in the 1930s and 1940s, and ordered the UK's first mainline diesel locomotives, 10000 and 10001. The London, Midland and Scottish Railway (LMS1) was a British railway company. ...

Numbering

When the LMS was formed, it undertook a complete renumbering of all locomotives so that all locomotives in the same class were together. The following broad series were used:

 1-4999: Ex-Midland Railway 5000-9999: Ex-London & North Western Railway 10000-12999: Ex-Lancashire & Yorkshire Railway and Furness Railway 14000-17999: Ex-Caledonian Railway, Highland Railway and Glasgow & South Western Railway 

Later, it was decided to use the 1-9999 series for the more modern locomotives, with older pre-Grouping types taking numbers from 10000 upwards. This involved renumbering both new and old locomotives to put them in the appropriate sequence. In some cases, the renumbering of old locomotives was simplified by just adding 20000 to the old number. In particular, this happened to a lot of the ex-LNWR locomotives that survived into the 1930s.

Classification

Locomotives were given a two-part classification. The first digit was a number between 0 and 9, which represented the power of the locomotive, with 0 being the least powerful and 9 the most powerful. The number was followed by a code indicating what type of traffic the locomotive was used for, i.e. F for Freight, P for Passenger, and MT for Mixed Traffic. The one downside was that the classification did not distinguish between different types of locomotive, so many very different types would have been classified '4F' for instance.

London & North Eastern Railway (LNER)

The LNER used mainly steam locomotives for haulage, despite the fact that it inherited the North Eastern Railway's interests in DC overhead electric traction. Its later plans to introduce 1500 V DC overhead schemes on the Woodhead route over the Pennines and the London Liverpool Street-Shenfield suburban line were interrupted by World War II, and completed only after Nationalisation. The London and North Eastern Railway or LNER was the second-largest of the Big Four railway companies created by the Railways Act 1921 in Britain. ...

Numbering

When the LNER was created in 1923, it adopted a simple renumbering process to ensure that no numbers were duplicated amongst its inherited locomotives, as follows:

 1-2399: Ex-North Eastern Railway (numbers unchanged) 2400-2999: Ex-Hull & Barnsley Railway (renumbered without reference to original number) 3000-4999: Ex-Great Northern Railway (3000 added to original number) 5000-6799: Ex-Great Central Railway (5000 added to original number) 6800-6999: Ex-Great North of Scotland Railway (6800 added to original number) 7000-8999: Ex-Great Eastern Railway (7000 added to original number) 9000-10999: Ex-North British Railway (9000 added to original number) 

New locomotives were numbered in blank gaps within these series. The result of this approach was that numbers were scattered throughout the list without reference to class, let alone type of locomotive. In 1942, a limited renumbering was proposed to bring the numbers of locomotives in some classes together, but that was superseded in 1946 by a complete renumbering of all locomotives, so that classes would be brought together and placed in a series with classes of the same type. The system was set up as follows:

 1-999: Express passenger tender (4-6-2 and large 2-6-2) locomotives 1000-1999: Large mixed-traffic tender (4-6-0, 2-6-0 and small 2-6-2) locomotives 2000-2999: Medium mixed-traffic tender (4-4-0 and 4-4-2) locomotives 3000-3999: Large freight tender (2-8-0 and 0-8-0) locomotives 4000-5999: Small mixed-traffic tender (0-6-0) locomotives 6000-6999: Electric locomotives 7000-7999: Passenger tank (Four coupled & 2-6-2T) locomotives 8000-8999: Shunting tank (steam and diesel) locomotives 9000-9999: Miscellaneous tank locomotives 10000: Experimental Class W1 (retained its pre-1946 number) 

Classification

Steam locomotives were classified according to their wheel arrangement based on the Whyte Notation, with each style of wheel arrangement being represented by a letter from A to Z, as follows: A selection of early 20th century locomotive types according to their Whyte notation and their comparative size The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early 20th century. ...

 A: 4-6-2. B: 4-6-0. C: 4-4-2. D: 4-4-0. E: 2-4-0. F: 2-4-2. G: 0-4-4. H: 4-4-4. J: 0-6-0. K: 2-6-0. L: 2-6-4. M: 0-6-4. N: 0-6-2. O: 2-8-0. P: 2-8-2. Q: 0-8-0. R: 0-8-2. S: 0-8-4. T: 4-8-0. U: 2-8-0+0-8-2. V: 2-6-2. W: 4-6-4. X: 4-2-2 or 2-2-4. Y: 0-4-0. Z: 0-4-2. 

After the letter was a number, which was unique to a particular type of locomotive. In general locomotives with tenders received lower class numbers (A1, B1 etc) and tank locomotives received higher class numbers (D50, J50 etc).

Diesel and electric locomotives were given three-part classifications, commencing with either 'DE' for Diesel (electric transmission), 'DM' for Diesel (mechanical transmission) or 'E' for Electric, then 'B' for Banking, 'E' for Express, 'M' for Mixed Traffic or 'S' for Shunter, and finally a number issued to different types consecutively from 1.

British Railways (BR)

This section also covers the post-privatisation period, since the broad numbering and classification arrangements have not altered since the break-up of British Rail.

1948 Numbering and Classification

Immediately after Nationalisation, BR had to decide how to number and classify the stock it had inherited from the 'Big Four' companies, and how newly-built stock would be included. In the main, it decided to simply adapt what was already there.

Locomotives

The classification systems of the 'Big Four' were left unchanged for the inherited locomotives. However, BR decided to adopt the former LMS method of classification (see above) as its preferred model. All inherited locomotives received a classification in this series as well as their traditional classification.

In order to remove the duplications of locomotive numbers, all locomotives were placed into a new number series as follows:

This approach meant that the numbering arrangements adopted by the pre-Nationalisation companies were retained in the new system, and new locomotives built to the designs of the old companies were numbered appropriately in their series. The new series for diesel, petrol, gas and electric locomotives were arranged as follows:

 10xxx: Mainline diesel Locomotives 11xxx: Diesel mechanical & diesel hydraulic shunters (British Railways orders) 12xxx: Ex-LMS shunters 300 hp (224 kW) and over 13xxx: Diesel electric shunters (British Railways orders) * 150xx: Ex-LNER shunters 151xx: Ex-GWR shunters 152xx: Ex-SR shunters 18xxx: Gas turbine locomotives 20xxx: Ex-SR electric locomotives 26xxx: Ex-LNER electric locomotives 

Note: * Number 13000 was initially allocated to a 250 hp (186 kW) ex-LMS shunter, but this locomotive was withdrawn before being renumbered.

Modern Traction

Numbering of electric multiple units was undertaken on a regional basis, with each region having its own series commencing from 001. Most diesel multiple units were not allocated unit numbers, though in later years numbers were allocated on a regional basis in the Scottish and Western regions, and by individual depots elsewhere. For more details on these series, see British Rail Regional Multiple Unit Numbering. A multiple unit is a passenger train whose carriages have their own motors, either diesel (DMUs) or electric (EMUs), and do not need to be hauled by a locomotive. ... A diesel multiple unit can be: a multiple unit powered by a diesel engine; one or more of these form a passenger train; a combination of diesel powered locomotives operating under the control of one engine-driver, coupled together consecutively, usually at the head end of the train, which further... Prior to the introduction of TOPS, electric multiple unit numbers were allocated by the British Rail regions in their own series. ...

To classify electric stock, a two-letter prefix was used to indicate what type it was, followed by a number issued sequentially from 1. This was a system adapted from that used by the LNER for its electric stock (e.g. EM1, see above). The additional prefixes used were:

 AL: AC electric locomotive AM: AC electric multiple unit 

The Eastern and North Eastern Regions used a series of different classifications for diesel locomotives. The first method was adapted from the old LNER system, and applied only to diesel shunters operating on those regions. It comprised a prefix, indicating the transmission type and wheel arrangement, followed by a number allocated sequentially from 1. The prefixes used were:

 DEJ: Diesel-electric, 0-6-0 wheel arrangement DJ : Diesel-mechanical or hydraulic, 0-6-0 wheel arrangement DY : Diesel-mechanical or hydraulic, 0-4-0 wheel arrangement 

The second classification system was developed in 1955 and applied to all types then in existence on British Rail. It was based on the format Dx/y, where x was the power of the locomotive in hundreds of horsepower and y was a number allocated sequentially to specific types (e.g. D1/1 would be a shunter with a 100 to 199 hp (75 to 148 kW) rating, D33/1 would be a Type 5 locomotive). The series was extensively re-arranged in 1962 in a somewhat confusing way, but following the same basic principles.

The Southern Region followed existing numbering (for its multiple units) and classification systems (for both multiple units and locomotives) inherited from the SR. The EMU classification system (e.g. 4-Sub, as described above) system was also extended to include their diesel-electric multiple units, but with a single letter. Locomotives were given a two-letter code, with the second letter indicating detail differences within the main type. For instance, what were to become Class 33 locomotives from 1973, were Class KA or KB depending on whether they were the standard design or that fitted for push-pull working with 4TC units. A classic Belgian multiple unit of type 74 A multiple unit is a passenger train whose carriages have their own motors, either diesel (DMUs) or electric (EMUs), and do not need to be hauled by a locomotive. ...

1957 Numbering and Classification

In the early period of British Railways, steam locomotion had continued to be of prime importance. Apart from a growing selection of diesel shunters, there had been almost no development of mainline diesel or electric traction beyond the few locomotives ordered by the 'Big Four' companies.

The 1955 Modernisation Plan heralded big changes in this situation, and from 1957 a new numbering system was used for diesel and electric locomotives ordered by British Railways, including those shunters ordered before 1957, which were renumbered into the new system. All steam and gas-turbine locomotives, and diesel and electric locomotives built to pre-nationalisation orders retained their existing numbers under the 1948 arrangements, though some had a 'D' or 'E' prefix added to their number in error.

Diesel Locomotives

The numbering system matched up the new 'Type' classification that had been adopted for diesel traction, and which was based on the power of the locomotive. The broad categories were as follows:

 D1-D1999 - Type 4 - 2000 to 2999 hp D2000-D2999 - Shunters - Under 300 hp D3000-D4999 - Shunters - 300 to 799 hp D5000-D6499 - Type 2 - 1001 to 1499 hp D6500-D7999 - Type 3 - 1500 to 1999 hp D8000-D8999 - Type 1 - 800 to 1000 hp D9000-D9999 - Type 5 - Over 3000 hp 

Of the shunters that were renumbered, the only transparent renumbering was for those that had been numbered in the 13xxx series, for which the '1' was simply replaced with a 'D'. All the others were completely renumbered to separate out the different classes.

As always, there were some oddities. The 650 hp diesel hydraulic locomotives later Class 14 were numbered from D9500 upwards. When the Type 2 series got too crowded later on, new Sulzer Type 2 locomotives (later Class 25) were numbered from D7500 upwards. Some experimental locomotives carried D0xxx numbers.

When the last mainline steam locomotive was withdrawn in August 1968 (leaving only three on the self-contained narrow gauge Vale of Rheidol Railway), the 'D' prefix was dropped.

Electric Locomotives

The numbering system was divided into two series, one for AC locomotives and one for DC lomotives. DC locomotives were numbered from E5000 upwards, and DC electro-diesel locomotives, with a diesel generator for working off electrified lines, were numbered from E6000 upwards. An Electro-diesel locomotive is a special type of locomotive that can be powered either from an external electricity supply (i. ...

For AC locomotives, the first number was to be an indication of power. For example, if the power was in the range 2000 to 2999 hp, it would be numbered between E2000-E2999, and so on. In fact, apart from E2001 (the prototype AC locomotive, converted from a gas turbine locomotive, later Class 80), all AC locomotives were numbered from E3001 upwards. When new 5000 hp locomotives were under construction (later Class 87), these were allocated numbers from E3201 upwards, though they never carried these numbers as the 1973 arrangements were already in place by the time the first one was built.

1973 Numbering & classification - TOPS System

At the end of the 1960s, British Railways adopted the Total Operations Processing System (TOPS), a computerised system developed by the Southern Pacific Railroad in the United States . All types of locomotive and multiple unit received a TOPS classification, according to this broad division: This article is about the railroad stock management system TOPS. For the DEC PDP-10 operating systems, see TOPS-10 and TOPS-20 The Total Operations Processing System, better known by its initials TOPS, is a computer system for managing the locomotives and rolling stock (railroad cars) owned by a... The Southern Pacific Railroad (AAR reporting mark SP) was an American railroad. ...

 0xx: Locomotives and Ships 1xx: Diesel-Mechanical (including hydraulic) Multiple Units 2xx: Diesel-Electric Multiple Units 3xx: AC and Multi-Voltage Electric Multiple Units 4xx: Southern Region DC Electric Multiple Units 5xx: Other DC Electric Multiple Units 9xx: Departmental (non-revenue earning) Multiple Units 

From 1973, British Railways started to apply new numbers to locomotives and multiple units based on the TOPS classification system. The format of these numbers is xxxyyy, where xxx is the class number and yyy the unique identifier for that locomotive or unit. All locomotive classes have unique identifiers that commence at xx001, except classes 43 (originally treated as multiple unit carriages), 97 and 98 (departmental and steam locomotives). Multiple unit classes are treated differently, because an attempt has been made to give units working within the same region or sector unique identifiers (for more information see British Rail Regional Multiple Unit Numbering). In recent years, unit numbers have also been tied in with the numbers of the carriages within a unit (e.g. 150201 is formed of carriages 52201 and 57201). As a result, very few multiple unit classes commence from xxx001. Prior to the introduction of TOPS, electric multiple unit numbers were allocated by the British Rail regions in their own series. ...

Where there are variations within a class, subclasses are used in the format xxx/y. Usually, the subclass is connected to the first digit of the unique identfier, so that the first locomotive in subclass 47/3 was 47301. However, some caution is required on this point for the following reasons:

• Where there are more than 100 examples in a subclass the identifier becomes obscured, e.g. 31201 is in subclass 31/1, which runs from 31101 to 31327.
• Some classes renumbered from the 1957 arrangements simply had consecutive numbers from xx001, which ignored the subclasses identified, e.g 25033 was in subclass 25/1.
• Where numbers within a class became congested, it wasn't always possible to make the connection, e.g. locomotives in subclass 47/2 were numbered downwards from 47399. In other cases it wasn't possible to start from xxxy01, e.g. subclass 08/9 started from 08991 because subclass 08/0 ran from 08001 to 08958.
• Because of the different arrangements for numbering multiple units (see above), subclasses may have no link to the unique identifier at all; this is especially true on the Southern Region. Sometimes, where there is a principal run of units and a small variant subclass, the majority will be subclass xxx/0 whatever their identifier, but the variant's subclass does reflect the identifier, e.g. Subclass 302/0 for the standard passenger units (302201-312) and 302/9 for the postal units (302990-302993).

So far as renumbering from the 1957 arrangements was concerned, most locomotives retained the last two digits of their number, though some classes were renumbered without reference to their previous numbers (notably Classes 45 and 86, which were part-way through a programme of fitting electric heating apparatus to a selection of locomotives). The other exception was the first locomotive of each class, which had usually carried Dxx00 numbers under the 1957 arrangements, since TOPS could not handle numbers ending in '000'. These were often renumbered to the end of the class (e.g. D400 became 50050), or took the number of another class member that had already been withdrawn (e.g. D5000 became 24005).

The situation was different for multiple units. Where unit numbers were carried, they were usually three-digit already. TOPS simply prefixed these existing unit numbers with the newly-allocated TOPS class number. The process was more complicated on the Southern Region, which used four-digit unit numbers and where a more general unit renumbering was required so that the first number of the unit coincided with the last digit of the class number. Many diesel multiple units were not kept in regular formations, so did not have existing unit numbers, and this situation was not changed under TOPS (except following refurbishment).

The TOPS system has been perpetuated by the privatised railway, though the allocation of classes and numbers appears to have become more random and less governed by the rules followed by British Railways (but even they made exceptions). See British Carriage and Wagon Numbering and Classification for an explanation of how TOPS applied also to carriages and wagons. A number of different numbering and classification schemes have been used for carriages and wagons on Britains railways, and this page explains the principal systems that have been used. ...

Please Note: This section explains the successful application of TOPS to multiple unit stock, the arrangement that persists today. However, there was an earlier attempt to apply TOPS that differed from the arrangement set out below. More details about the first arrangement may be found here. This article is about the railroad stock management system TOPS. For the DEC PDP-10 operating systems, see TOPS-10 and TOPS-20 The Total Operations Processing System, better known by its initials TOPS, is a computer system for managing the locomotives and rolling stock (railroad cars) owned by a... At the end of the 1960s, British Railways adopted the Total Operations Processing System (TOPS), a computerised system developed by the Southern Pacific Railroad in the United States. ...

Locomotives

The series of locomotive classes were allocated according to the following pattern:

 01-69: Diesel locomotives 70-79: DC electric and electro-diesel locomotives 80-96: AC and multi-voltage electric locomotives 97: Departmental (non-revenue earning) locomotives 98: Steam locomotives 99: Ships 

The following table shows in more detail how the classification of diesel locomotives was overlaid on the 1957 classification by Type:

 01-07: Shunters - Under 300 hp - Ex D2000-D2999 08-14: Shunters - 300 to 799 hp - Ex D3000-D4999/D9500-D9999 15-20: Type 1 - 800 to 1000 hp - Ex D8000-D8999 21-32: Type 2 - 1001 to 1499 hp - Ex D5000-D6499/D7500-D7999 33-39: Type 3 - 1500 to 1999 hp - Ex D6500-D7499 40-54: Type 4 - 2000 to 2999 hp - Ex D1-D1999 55-69: Type 5 - Over 3000 hp - Ex D9000-D9499 

Ac electric locomotive classes AL1 to AL6 became 81-86 in order.

Multiple units

The multiple unit series were divided up as follows:

 100-114: 'Low Density' passenger units (i.e. few doors per carriage) 115-127: Mixture of 'High Density' (i.e. doors to every seating bay) and 'Cross-Country' (long distance) passenger units 128-131: Parcels units 140-144: Second generation railbus units ('Pacer') 150-199: Second generation units ('Sprinter', 'Networker', 'Turbostar', 'Coradia') 200-207: First generation units 210-249: Second generation units 250-299: Express units 300-312: First generation units 313-369: Second generation units 370-399: Express units 920-935: Southern Region departmental units 936-939: Other departmental electric multiple units 950-960: Other departmental diesel multiple units 

AC electric multiple units AM1-AM11 became 301-311 in order (in fact the AM1 units had already been withdrawn, so Class 301 was never actually used). The 1xx and 2xx series were originally arranged so that driving motors, driving trailers and trailer cars all had their own individual class numbers (presumably because these units were more prone to being reformed), but this was subsequently revised so that each type of unit had a single class number, as allocated to the driving motor car.

Whereas within most ranges class numbers were allocated sequentially as new types were constructed, the Southern Region adopted a more complicated system for their electric multiple units, with the second and third digits indicating in more detail the type of unit. Second digits were allocated as follows:

 40x: Southern Railway-designed units 41x: 1950s British Railways-designed units 42x: 1960s British Railways-designed units 43x: 1967 Bournemouth Electrification units 44x: 1970s British Railways-designed units 45x: 1980s British Railways-designed units 46x: 1990s Networker units 48x: Underground ('Tube'-sized) units (also temporary formations and, later, 'Gatwick Express' units) 49x: Unpowered trailer units (later 4x8) 

Third digits were allocated as follows:

 4x0: Express units with buffet (later 4x2) 4x1: Express units 4x3: Four-car outer-suburban units 4x4: Two-car outer-suburban units 4x5: Four-car inner-suburban units 4x6: Two/three-car inner-suburban units 4x7: Special purpose units (e.g. first 'Gatwick Express' units) 4x9: Single car units 

Of course, many exceptions arose over time. One major change was to change the classification of unpowered trailer units from 49x numbers to 4x8 numbers (which involved reclassifying Class 491 to Class 438). When Southern Region unit numbers were changed to fit with the TOPS classification system, former 4x0 classes were all reclassified to 4x2. This was necessary because Southern Region units only displayed the last four digits of their six-digit TOPS number, and it was decided that no painted unit number should commence with a '0'.

See also

For further information on individual classes of locomotive or multiple unit, see these pages:

• List of British Rail classes
• Steam locomotives of British Railways
• British Rail Regional Multiple Unit Numbering
• Locomotives of the Great Western Railway
• Locomotives of the Southern Railway
• SR Multiple Unit Numbering and Classification
• Locomotives of the London, Midland and Scottish Railway
• Locomotives of the London and North Eastern Railway