Of the four precursor railways, the LNER had perhaps the most developed system of classification for its locomotive fleet. This featured a letter to indicate the wheel arrangement followed by a sequential number for successive designs. For example, J72 was the (nominally) 72nd design of 0-6-0 loco. Some classes had sub-divisions, indicated by a slash and a further sequential digit (eg D17/1), while class codes weres sometimes re-used, even if examples of the earlier use were still in service. For example, when the LNER designed its 'standard' types, these were generally given the lowest numbers (eg B1), with the original uses of each code being given a vacant higher number (B18) ^[1]

The LMS used a system based on the power output of the loco and its intended use. The power was indicated by a digit from 0 to 9 (where 0 was the lowest powered and 9 the highest - although the 9 was never used by the LMS), and the use by one or two letters (P for passenger, XP for express passenger, F for freight, MT for mixed traffic). The power rating was based on different scales for passenger and freight traffic, and this resulted in some designs falling into two categories. These would both be shown in the classification (eg 5P4F) ^[2]. The main drawback of the LMS system was that some codes (such as 4MT and 3F) covered numerous different designs. Even so, it was adopted by British Railways for its new-build 'standard' steam loco classes (ranging from 2MT up to 9F), and was also applied to locos inherited from the other three companies.

The GWR and SR did not appear to have a coherent system of classification for locos, the types being distinguished by reference to their stock numbers (eg GWR 57xx class) or by the names they were given (eg GWR 'Castle' class and SR 'West Country' class).

Under the Modernisation Plan of 1955, British Railways started developing new designs of diesel locos, classified according to their power output. Type 1 locos were those of up to 1000hp, type 2 up to 1500hp, type 3 up to 2000hp, type 4 up to 3000hp and type 5 above 3000hp. Naturally, each type group could cover many different designs, and there was no offfical way of distinguishing these. Individual classes were often informally referred to by their type and make (eg Brush Type 2), by the names they carried (eg Peaks, Warships, Westerns) or by other aspects of their design or appearance (eg Deltics, Hymeks).

Electric locos were rare until the end of the 1950s. Those used on the Manchester-Sheffield line were ordered by the LNER and retained their classifications of EM1 and EM2 (for Electric Loco, Mixed-Traffic), as did a couple of vintage locos used for shunting on the Newcastle quayside line (ES1). The three prototype locos built by the SR similarly kept their classification of CC (based on the Co-Co wheel arrangement). BR introduced new electric locos to the Southern Region from 1958, followed by electro-diesels in 1962, these having no specification classification. When AC electrification was introduced to the London Midland Region from 1959, the new-build locos, of 5 different types, were simply classified A (for passenger work) or B (for freight/mixed-traffic) ^[3], though this was soon changed to classes AL1 to AL5 (presumably at the same time as the multiple units were classified AM1 etc).

In terms of technical classifications, little is known of any systems used by the big-four railway companies. Various diagram books can be viewed/downloaded on the Barrowmore Model Railway Group website. and these suggest that BR standard steam locos were not given any specific classfications (the diagrams only refer to the class). On the other hand, there is also a diagram book of ex-LMS locomotives, and each diagram has a number in the format EDnnn (for example, the 'Duchess' class are to diagram ED261D). Diagram books for BR diesel and electric locos show that there was a system of numbering for these. The books available on the BMRG website date from the early 1970s and include the TOPS class codes but not TOPS diagram/design codes. This suggests that the latter were introduced some time after the former. The diagram numbering is in the format of AA/xxyy/zz, where AA are one or two letters to indicate the power source (and transmission), xx are one or two digits to indicate the power rating (in approximately 100s of horsepower), yy are two digits to show the sequential design and zz are one or two digits to show variants. Thus DH/206/4 was the fourth variant of the sixth design of c.200hp loco, and it had hydraulic transmission. The sequential designs were within power rating, irrespective of transmission. Hence the 200hp bracket inlcuded DM200, DM201, DM202, DM203, DM204, DM205, DH206, DE207, DH208, DE209 and DM210 (all with at least a /1 variant). What became class 47 was assigned diagrams DE/4003/1 to DE/4003/21. For a list of the known diagram numbers, go to my Design List page, and select 'BR Pre-TOPS Loco Diagram' from the Design Series drop-down.