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:
|(0)xx||Locomotives and Ships (leading zero was ignored)|
|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.
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. This was usually where sub-classes had already been or were in the process of being created (for example Classes 45 and 47), but some classes were renumbered randomly for no apparent reason (for example Class 86). 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.
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.
Read more about this topic: British Rail Locomotive And Multiple Unit Numbering And Classification, 1973 Numbering and Classification
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