invite bids for twenty locomotives of the new Type 4 specification using the Brush electrical equipment intended for the cancelled order. Brush won the contract. This initial build of 20 locomotives (Nos. D1500 to D1519) were mechanically different from the remainder of the type, using Westinghouse-supplied brake systems, and would be withdrawn earlier than the rest of the class which used Metcalfe-Oerlikon brakes. Based on the success of these initial 20 locomotives an order for 270 was made, which was later revised upwards a number of times to reach the final total of 512. 310 locomotives were constructed by Brush in Loughborough, and the remaining 202 at BR's Crewe Works. Five locomotives, Nos. D1702 to D1706, were fitted with a Sulzer V12 12LVA24 power unit and classified as Class 48s; the experiment was not deemed a success, and they were later converted to standard 47s. Orders The locomotives were ordered in 6 batches as follows: D1500–D1519, 28 February 1961. D1520–D1681, 1 January 1962. D1682–D1706, 4 September 1962. D1707–D1781, 28 September 1962. D1782–D1861, 22 July 1963. D1862–D1999 & D1100–D1111, 24 March 1964. In service The first 500 locomotives were numbered sequentially from D1500 to D1999, with the remaining twelve being numbered from D1100 to D1111. The locomotives went to work on passenger and freight duties on all regions of British Rail. Large numbers went to replace steam locomotives, especially on express passenger duties. The locomotives, bar a batch of 81 built for freight duties, were all fitted with steam heating boilers for train heat duties. The initial batch of twenty, plus D1960 and D1961, were also fitted with electric train heating (ETH). With this type of heating becoming standard, a further large number of locomotives were later fitted with this equipment. In the mid 1960s, it was decided to de-rate the engine output of the fleet from to , significantly improving reliability by reducing stresses on the power plant, whilst not causing a noticeable reduction in performance. Sub-classes In the early 1970s, the fleet was renumbered into the 47xxx series to conform with the computerised TOPS systems. This enabled a number of easily recognisable sub-classes to be created, depending on the differing equipment fitted. The original series were based on train heating capability and were as follows; Class 47/0: Locomotives with steam heating equipment. Class 47/3: Locomotives with no train heating. Class 47/4: Locomotives with dual or electric train heating. However,