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, but the first attempt at applying TOPS was soon modified. This page explains the first attempt at using TOPS and cross-refers the classes allocated with those adopted in the successful re-arrangement. An explanation of the final arrangements for TOPS classification may be found here. Locomotives The first attempt to apply TOPS differed from the latter approach in two significant ways: xx/0 was not a valid subclass number. When the second arrangement was applied, original subclasses xx/1 became xx/0, xx/2 became xx/1 and so on. Some detail differences within classes were considered sufficient for a different class number to be allocated, rather than indicate it with a subclass. In addition, there are a number of cases where conversions led to complete reclassification, where similar circumstances later would be indicated simply by allocating a new subclass. The change in approach led to some odd gaps in the list of locomotive classes when they were re-arranged, as the principal class numbers were retained unaltered. Classes affected by this change in approach include: Class 29 (re-engined Class 21) Class 31 (re-engined Class 30) Class 34 (became Class 33/1) Class 43 (North British Locomotive-built Class 42 with different make of engine and torque-converter) Class 48 (Class 47 with experimental Sulzer vee-formation engine; later rebuilt to standard class 47 with in-line twin-bank engine) Class 72 (became Class 73/0) Diesel Multiple Units The first attempt to apply TOPS to diesel multiple units saw each carriage within a unit being allocated its own unique class number. This was probably because most DMUs were not kept in permanent formations. Classes were allocated according to the following division: 100-131: Driving Motor cars 140-150: Driving Trailer cars 160-190: Trailer cars 201-203: DEMU Driving Motor cars 211-212: DEMU Driving Trailer cars 221-225: DEMU Trailer cars 251: Express DEMU Driving Motor cars 261: Express DEMU Trailer cars However, this arrangement was later revised so that each unit adopted the class number previously given just to the main type of Driving Motor car (the exception being the DEMU units, which were completely changed). The table below cross-references the old and new TOPS classes: Electric Multiple Units The first attempt to apply TOPS to the Southern