this numbering system was later disrupted as locomotives were fitted with extra equipment and were renumbered into other sub-classes. For an overview of the renumbering see the List of British Rail Class 47 locomotives. This section summarises the main sub-classes that were created. Class 47/0 Originally numbered D1520–1781, D1837–74, D1901–59, D1962–99 and D1100 (later given TOPS numbers from 47001 to 47298), these locomotives were the "basic" Class 47 with steam heating equipment fitted. In the 1970s and 1980s, with steam heating of trains gradually being phased out, all locomotives fitted with the equipment gradually had their steam heating boilers removed. Some were fitted with ETH and became 47/4s, whilst the others remained with no train heating capability and were therefore used mainly on freight work. In the 1990s, the class designation 47/2 was applied to some class 47/0s and class 47/3s after they were fitted with multiple working equipment. The locomotives involved also had their vacuum braking systems removed or isolated, leaving them air braked only. This was mainly a paper exercise, however, and the locomotives were not renumbered; in this article they are included in Class 47/0. Class 47/3 Originally numbered D1782–1836 and D1875–1900 (later given TOPS numbers from 47301 to 47381), this sub-class was originally built with no train heating equipment and therefore remained as freight locomotives almost exclusively for their working lives. They were all fitted with slow speed control for working MGR coal trains (as were a number of Class 47/0s). However, during the summer months when train heat was not required, 47/3s could regularly be found hauling the extra trains that the holiday season brought. The sub-type remained stable until withdrawals started, although an "extra" 47/3, 47300, was created in 1992 when 47468 had its train heating equipment removed and was renumbered. This was a direct replacement for collision damaged 47343. Also, 47364 was renumbered to 47981 in 1993 for use on RTC test trains. Class 47/4 The designation for standard locomotives fitted with ETH and therefore used for passenger, mail and parcels use. The original batch of twenty locomotives (D1500–D1519) were built with dual steam and electric train heating, the electric heating being tapped from the locomotive's DC generator. However, no more were constructed in this way until 1967 when the final two Brush-built locomotives, D1960 and D1961, were fitted with a new type of electric heat system using an AC alternator. Approximately 112