being renumbered 20228 at the end of the class (which was not divided into sub-classes) it became 20050, displaced D8050 became 20128, and D8128 in turn became 20228. 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 privatisation of British Rail, though the allocation of classes and numbers appears to have become more random and less governed by the rules followed by British Rail (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. Locomotives The series of locomotive classes were allocated according to the following pattern: The following table shows in more detail how the classification of diesel locomotives was overlaid on the 1957 classification by Type: AC electric locomotive classes AL1 to AL6 became 81-86 in order. Class 01 was subsequently reused to register any ex-industrial/Ministry of Defence shunters used on the national network regardless of horsepower. Multiple units The multiple unit series were divided up as follows: AC electric multiple units AM1-AM11 became 301-311 in order (in fact the AM1 units had already been withdrawn, so Class 301 was never actually used).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