page_title: TOPS
section_title: TOPS numbering under British Rail
hierarchical_section_title: TOPS / Adoption by British Rail / TOPS numbering under British Rail
caption_attribution_description: English: The dateplate of a British Rail Class 31/4 diesel locomotive.
context_page_description: Total Operations Processing System is a computer system for managing the locomotives and rolling stock owned by or operated on a rail system. It was originally developed by the Southern Pacific Railroad, the Stanford University and IBM and was introduced on the railroad in 1968; despite being widely sold to the United States railroads, the system found greater success in the United Kingdom, where it has been used by British Rail and its successors.
  Southern Pacific itself developed a newer system called the Terminal Information Processing System, which replaced TOPS in 1980.
context_section_description: Sequentiality was all that was required, but with the requirement to renumber, it was decided to adopt a logical system for classification, and the five- or six-digit TOPS number was divided into two parts. No class of locomotive or multiple unit numbered over 1000 examples, so the last three digits were used for the individual number between 001 and 999 (Although  Class 43 goes down to 000, it being the HST prototype power cars) in that class. The first two or three digits were used to denote the class of locomotive or multiple unit. The numbers were often written in two space separated groups, such as "47 401" to highlight that division, but the TOPS system actually stored and displayed them without the space: "47401". Sub-classifications were indicated in the TOPS system with a slash and a subclass number, e.g. "47/4". It was convention, though not enforced within the TOPS system, that subclass numbers were boundaries in the locomotive numbering system, such that class "47/4" started with number "47 401". If there were more than 99 numbers in a subclass, the number series extended to the next value of the third digit; thus, since there were more than 200 locomotives in class "47/4", subclasses "47/5" and "47/6" did not exist, and the next valid subclass by convention was "47/7" starting with "47 701".  However, in some cases, the sequences do not match, e.g. 158/0 numbers start at 158 701.
  Locomotives are assigned classes 01–98: diesel locomotives 01–79 (originally 01–69),  AC electric locomotives 80–96, departmental locos (those not in revenue-earning use) 97, and steam locomotives 98. DC electric locomotives were originally allocated classes 70–79 but this was modified in 2011 (see British Rail locomotive and multiple unit numbering and classification); the sole relic of this is Class 73 which continues unrenumbered, probably because it can be considered equally a diesel locomotive as it is a DC electric. One oddity was the inclusion of British Rail's shipping fleet in the system as Class 99. Diesel multiple units (DMUs) with mechanical or hydraulic transmission are classified 100–199, with electric transmission 200–299. Electric multiple units (EMUs) are given the subsequent classes; 300–399 are overhead AC units (including AC/DC dual-voltage units, and new DC only units with pantogaph wells allowing for conversion to AC), while Southern Region DC third rail EMUs are 400–499, other DC EMUs 500–599. More recently, new electric multiple units from the Siemens Desiro family have been given the 700 series and new units from the Hitachi A-train family have been given the 800 series. Selected numbers in the 900 series have been used for departmental multiple units, mostly converted from former passenger units.  
  Coaching stock and individual multiple unit cars are allocated five-digit numbers; since the early 1980s, it has been forbidden for them to have the same numbers as locomotives, but before then duplication was possible because they carried a prefix letter, which was considered part of the number. More recent EMU deliveries have six-figure coach numbers.
caption: Brush Type 2 locomotives became Class 31 under TOPS. This is the data panel from a Class 31/4; the 31/4 subclass being used for locomotives with Electric Train Heating.
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