The numbering of locomotives by the 'big four' companies (GWR, LMS, LNER and SR) is a broad and complex subject that is best covered by the various specialised websites and books. Our coverage starts with the formation of British Railways on the first day of 1948. Some way of differentiating the items of inherited locos, many of which had duplicated numbers, was needed. There was an initial plan to use prefix letters, as was applied to coaches and wagons, with at least some ex-LNER locos gaining an E prefix. However, the plan finally adopted was to amend the numbers, although the way this was implemented could be considered to be akin to the use of numerical prefixes.

The Great Western Railway (GWR) was unique amongst the big four in using cast numberplates for its loco fleet. As such, renumbering or pre-fixing would have required these plates to be modified or dispensed with. It was decided instead to let ex-GWR locos continue with their existing numbers. Former Southern Railway (SR) locos had 30,000 added to their existing numbers, with a few exceptions such as for types that had previously had letters in their numbers. The London Midland and Scottish Railway (LMS) had the largest loco fleet, and the numbering already ran into five-digit numbers. Most ex-LMS locos had 40,000 added to their numbers, taking up new numbers in the 4xxxx and 5xxxx series, again with a few exceptions. Finally, locos from the London and North Eastern Railway (LNER) had 60,000 added to their numbers.

This policy was applied to steam locos, but there were also a small number of diesel and electric locos in the fleets of the big-four. These were given new numbers in the 1xxxx and 2xxxx series respectively. The remaining series (7xxxx, 8xxxx and 9xxxx) were used for steam locomotives taken over from the War Department and for the new 'standard' design steam locos introduced by BR. These three series were not to see a great deal of use as, less than ten years after the formation of BR, the Modernisation Plan of 1955 proposed the adoption of diesel and electric traction to replace steam. A scheme was put forward for a new number series for diesel locos and this was implemented from May 1957.

The 1957 numbering scheme applied four-digit numbers to diesel locos, each with a D prefix, and grouped into blocks based on power rating as follows ^[1].

• D1 to D2499 were for Type C locos (over 2000hp)
• D2500 to D2999 for shunters of up to 300hp
• D3000 to D4999 for shunters of over 350hp
• D5000 to D7999 for Type B locos (1000 to 1250hp)
• D8000 to D8999 for Type A (800 to 1000hp)

Note that this scheme did not appear to cater for locos in the 1250 to 2000hp bracket. Later in the year, the power ratings were changed from A/B/C to 1-5 and the allocated number blocks amended as follows.

• D1 to D999 for Type 4 locos (2000 to 2500hp)
• D1000 to D1999 for Type 5s (3000hp and above)
• D2000 to D2999 for small shunters
• D3000 to D4999 for large shunters
• D5000 to D6499 for Type 2s (1000 to 1250hp)
• D6500 to D7999 for Type 3s (1500 to 1750hp)
• D8000 to D8999 for Type 1s (750 to 1000hp)
• D9000-D9999 for miscellaneous locos

Again there were gaps in the power brackets (eg 1250 to 1500hp and 2500 to 3000hp). The ranges were later amended when required for new deliveries (eg what became class 47s under TOPS had a power of 2750hp and were classed as Type 4s but were given numbers in the D1xxx range previously intended for Type 5s).

Several aspects of this scheme seem somewhat illogical, at least in retrospect. Firstly, the plan allowed for no more than 10,000 diesel locos to eventually replace the significantly larger number of steam locos. The planners must have decided this was adequate given that there would also be increased use of multiple units and electric traction. Secondly, the restriction to four digits meant that the new plan offered no more scope than the existing 1xxxx number series. Finally the allocation of number blocks was based on predictions for the future locomotive fleet requirements, some of which proved to be incorrect. Notably, the provision of 3,000 numbers for shunting locos turned out to be over-generous as changing operating practices reduced the need for them. Over 800 numbers in the D4xxx range were never used. Conversely, the quantity (and variety) of Type 1 and 2 locos built used up most of the allocated ranges and, as will be seen, some had to be given numbers 'out of range'.

Application of new numbers was started in May 1957, although the process was not to be finally completed until 1964 ^[1]. The majority of diesels in stock by 1957 were shunters and the few main-line locos (mainly prototypes such as the LMS-ordered 10001 and 10002) were not allocated new numbers, presumably as it was felt they would not be around for long. Some of the earliest shunters were also ignored, perhaps for the same reason. In the event, some of the locos in the 12xxx and 15xxx number series (by then classified as TOPS classes 11 and 12 respectively) lasted until the early 1970s. They were never re-numbered, although at least 4 erroneously gained D-prefixes to their five-digit numbers ^[1].

Small (under 300hp) shunters had been mainly numbered in a continuous series from 11100 upwards, and these were given new numbers in the D2xxx series. The renumbering kept batches in the same order, but types were grouped into blocks and there were few cases of the old and new numbers correlating. Larger shunters had been in the 13000 to 13366 range and these were changed directly to the D3000 to D3366 range.

The vast array of diesel locos built in the late-1950s and early-1960s were assigned D-prefixed numbers in the new series, usually in blocks starting at at a number ending in 00. Many of the blocks saw limited use (for example, there were only ever 5 locos in the D6xx block), while in other cases there were follow-on orders. For several types this lead to the situation were the number series 'ran into' a block that had already been used and, in such cases, non-contiguous blocks had to be allocated. This happened to the classes that later became 20 (D8000-D8199 had to be followed by D8300-D8327), 25 (D5151-D5299 followed by D7500-D7677), 31 (D5500-D5699 followed by D5800-D5862), 37 (D6700-D6999 followed by D6600-D6608) and 47 (D1500-D1999 followed by D1100-D1111). The Deltic locos for the East Coast Main Line were originally to have been numbered D1000-D1021, then changed to D1500-D1521. With the D1xxx range re-assigned to Type 4 locos, the Deltics ended up as D9000-D9021 in the former 'misceallaneous' series. Here they were later joined by the Swindon 0-6-0 hydraulics D9500-D9555. It is slightly ironic that the highest- and the lowest-powered main-line diesels were numbered in the same range.

Existing electric locos were left un-touched by the 1957 renumbering. The EM1 and EM2 types used between Manchester and Sheffield did later gain E-prefixes to their existing numbers 26000-26057 and 27000-27006, but this appears to have been an anomaly. New-build electric locos were assigned numbers in a new E-prefixed series, based on numbers of up to 4 digits, as with the D-prefixed series. Numbers from E5000 were for DC electric locomotives, the first appearing in 1958. The first AC loco was a rebuild of gas turbine loco 18100, also in 1958. Initially renumbered E1000, it was changed to E2001 in 1959 ^[2] . New-build AC locos followed shortly afterwards and were numbered from E3001 upwards. Initially it was planned that locos geared for freight traffic would be in their own E33xx series but, after a couple of applications, this was dropped and the locos renumbered back into the 'mixed-traffic' E30xx series. Further DC locos were delivered from 1962 but, as these also had diesel engines, they were numbered in a new E6xxx series.

With the ending of main-line steam on BR in 1968 it was realised that the D-prefix to diesel loco numbers was now no longer needed and could be dropped without the risk of creating duplicated numbers. The policy was duly implemented and locos receiving repaints appeared without prefixes (The only class of diesel loco being delivered at this time were the 50s, and these all came with D prefixes in place ^[3]). This 'renumbering' is largely ignored on this website (and in many other published sources) for several reasons. Firstly, the numbers were not actually changed, just the prefix. Secondly, there seems to be no record of which locos were de-prefixed and when. Thirdly, the process was never completed, with a more fundamental upheaval coming just a few years later (qv). Electric locos had to keep their E-prefixes as some of their numbers otherwise duplicated those in the diesel series. It was at this time that previously plain numbered electric locos in the 260xx and 270xx blocks gained E-prefixes. TOPS class codes had recently been introduced and thoughts were no doubt turning to a related renumbering exercise. Prefixing these electric locos freed up the 260xx and 270xx blocks for diesels though, as it turned out, the electrics had their numbers changed again before those blocks were re-used.

The introduction of TOPS
TOPS was a new computerised control system for BR rolling stock and was introduced in phases during the early 1970s. New TOPS class codes for locos had been introduced and refined in the period 1967 to 1969, bringing new order to the system. Each class was numbered in approximate order of the type's power rating. Class 01 was for the lowest powered shunters while class 55 was for the highest powered diesel. The new classes codes reflected the previous power ratings, such that classes that had been in the 'Type 4' power rating were now given class codes in the range 40-47 and so on. DC electric locos (and electro-diesels) were given classes starting at 70, with AC electric locos taking numbers from 80 upwards. A renumbering programme using these class codes as the basis was the next logical step, though it was not done straight away.

The plan was relatively simple. Locos would be given a 5-digit number comprised of the 2-digit class number followed by a 3-digit 'serial' number. The basic version of each class would be numbered from 001 upwards, with other variants given numbers starting at higher blocks. It was realised that the first digit of the serial could relate to the sub-class and this lead to some revisions to the class numbering plan in August 1973 ^[4]. Taking class 47 as an example, the basic variant was originally intended to be classified as 47/1, with other variants identified as 47/2, 47/3 etc. However, as the base type was now to be numbered from 47001 upwards, the sub-class for these was changed to 47/0. Similarly, what was to have been 47/2 became 47/3, this because there were more than 200 locos in class 47/0 so the next sub-class would start its numbers at 47301.

The first draft of the renumbering list was made in 1971 ^[5], and initially covered only electric locos. It also separated the class and the serial parts of the numbers with a dot (as in 81.001), though this was later dropped and the numbers were either applied in with a short space between the class and serial, or all joined together as one 5-digit number. The first actual renumbering was of Woodhead electric loco 26050, which became 76050 in November 1971 ^[6]. Various other locos in classes 76, 82, 83, 84 and 86 were renumbered in 1972.

Diesel locos were left until the beginning of 1973, when all diesel locos then in stock were allocated new numbers ^[7]. This plan ignored locos that had already been withdrawn, and made no attempt to relate the old number to the new number, other than that they were in the same order. In August 1973 the plan was revised, and (along with the amended sub-class codes mentioned above) took the opportunity to relate the new numbers to the old ones where possible. To give a couple of examples, the one hundred and twenty-three remaining locos of class 03 were originally going to be renumbered to 03001-03123. This was changed to numbers in the ranges 03004-03196 and 03370-03399, with the last three digits being the same as the previous D2xxx numbers. This explains the curiously large gap in the series, numbers D2200 to D2341 having been carried by Drewry shunters that were all withdrawn before the advent of TOPS (though which were allocated the class number of 04). The twenty-two 'Deltic' locomotives numbered D9000-D9021 were originally to have been renumbered to 55001-55022 in the same order (i.e. D9000 would become 55001). Again to preserve the last digits of the old numbers, they were in fact renumbered to 55022/001-021.

The policy of matching up digits of old numbers to those of the new numbers influenced the renumbering of several classes (notably 13, 40, 50) but there were other classes were a variation was adopted. These classes (20, 26, 37) had technical variations which, despite not warranting the creation of a separate number series, were thought to best if grouped together. Taking the class 20 as an example, they were originally numbered D8000-D8199 and D8300-D8327 (the D82xx series having been used for BTH Type 1s which were allocated TOPS class 15 but all withdrawn before the renumbering). Class 20 locos up to D8049 had a different type of traction motor to the later locos, while D8128 upwards had 4-character headcode boxes in place of disc headcodes. Neither of these distinctions needed to be recognised by the creation of a sub-class, but they were to influence the renumbering. D8001-D8049 became 20001-20049, with D8000 tacked on the end of this batch as 20050. This meant that D8050 had to become 20128, after the remaining disc-headcode locos D8051-D8127, which became 20051-20127. This in turn meant that D8128 had to be renumbered to 20228, afer the 4-character headcode locos D8128-D8199 and D8301-8327, which became 20128-20199 and 20200-20227 respectively.

The first diesel locos to actually carry TOPS numbers were those in class 45, which (uniquely for diesels) were numbered in a non-sequential way. Locos being modified with electric train heating were to be class 45/1 and the first became 45101 in March 1973, followed by others numbered in order of modification rather than related to their previous numbers. Twenty-three steam-heat 45s were then renumbered 45001-45023 in no apparent order. Once the total of fifty 45s to be given electric heating were identified, the remaining fifty-four locos were allocated new numbers 45024-45077 in the order of their original numbers. (On the subject of the first diesel to carry a TOPS number, there was a reference to D5518 having been renumbered 31101 in February 1973 ^[8] but this appears to have been a misprint).

One curiosity of the renumbering scheme was the way classes 25 and 26 were dealt with. Both of these classes had sub-classes, but the renumbering put all the locos into a continuous number series, whereas on other classes of locos, different sub-classes had their own number blocks. So, taking the 25s, locos in sub-class 25/0 were renumbered 25001-25025, 25/1s became 25026-25082, 25/2s became 25083-25247 and 25/3s became 25248-25327.

To be continued...