In 1973, the TOPS computer system for managing locomotives and rolling stock owned by a rail system, was introduced. Hauled rolling stock continued to carry numbers in a separate series. The adoption of the TOPS system made for some changes in the way the railway system in Britain worked. Hitherto, locomotives were numbered in three different series. Steam locomotives carried unadorned numbers up to five digits long. Diesel locomotives carried four-digit numbers prefixed with a letter 'D' and electric locomotives with a letter 'E'. Thus, up to three locomotives could carry the same number - steam loco 4321, diesel D4321 and electric loco E4321. TOPS could not handle this and it also required similar locomotives to be numbered in a consecutive series in terms of classification, in order that they might be treated together as a group. A new classification system was devised in which, for example, all Brush Type 4 locomotives were now called Class 47 and all had numbers beginning 47xxx.