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 and classification - TOPS At the end of the 1960s, British Rail adopted the Total Operations Processing System (TOPS), a computerised system developed by IBM, the Southern Pacific Transportation Company and Stanford University in the United States. All types of locomotive received a TOPS classification, and multiple units were later included according to this broad division: From late 1970, British Rail started to apply new numbers to locomotives and multiple units based on the TOPS classification system, the first classes to be dealt with being the LNER-design EM1 type (TOPS class 76) and the AL3 and AL4 types of AC electric locomotives (TOPS classes 83 and 84). 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 (High Speed Train power cars, originally classified as multiple-unit vehicles), 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. When the main renumbering scheme was published in 1973, many sub-classes were redesignated to better align the locomotives to be numbered in that sub-class with the sub-class number. Thus, class 47 was originally divided into sub-classes 47/1 (locomotives fitted with steam-heating equipment), 47/2 (not fitted with train-heating equipment) and 47/3 (fitted with electric train-heating equipment), but in 1973 these sub-classes were redesignated 47/0, 47/3 and 47/4. Usually, the subclass is connected to the first digit of the unique identifier, so that the first locomotive in subclass 47/3 was