within the main type. For instance, what were to become Class 33 locomotives from 1973, were Class KA or KB depending on whether they were the standard design or that fitted for push-pull working with 4TC units. 1957 numbering and classification In the early period of British Railways, steam locomotion had continued to be of prime importance. Apart from a growing selection of diesel shunters, there had been almost no development of mainline diesel or electric traction beyond the few locomotives ordered by the Big Four companies. The 1955 Modernisation Plan heralded big changes in this situation, and from 1957 a new numbering system was used for diesel and electric locomotives ordered by British Railways, including those shunters ordered before 1957, which were renumbered into the new system. All steam and gas-turbine locomotives, and diesel and electric locomotives built to pre-nationalisation orders retained their existing numbers under the 1948 arrangements, though some had a 'D' or 'E' prefix added to their number in error. Diesel locomotives The numbering system matched up the new 'Type' classification that had been adopted for diesel traction, and which was based on the power of the locomotive. The broad categories were as follows: Of the shunters that were renumbered, the only transparent renumbering was for those that had been numbered in the 13xxx series, for which the '1' was simply replaced with a 'D'. All the others were completely renumbered to separate out the different classes. As always, there were some oddities. The 650 hp diesel hydraulic locomotives later Class 14 were numbered from D9500 upwards. When the Type 2 series got too crowded later on, new Sulzer Type 2 locomotives (later class 25) were numbered from D7500 upwards. Some experimental locomotives carried D0xxx numbers. When the last mainline steam locomotive was withdrawn in August 1968 (leaving only three on the self-contained narrow gauge Vale of Rheidol Railway), the 'D' prefix was dropped. Electric locomotives The numbering system was divided into two series, one for AC locomotives and one for DC locomotives. DC locomotives were numbered from E5000 upwards, and DC electro-diesel locomotives, with a diesel generator for working off electrified lines, were numbered from E6000 upwards. For AC locomotives, the first number was to be an indication of power. For example, if the power was in the range 2000 to 2999 hp, it would be numbered between E2000-E2999, and so on. In fact, apart