The British Rail Class 47 or Brush Type 4 is a class of diesel-electric locomotive that was developed in the 1960s by Brush Traction. A total of 512 Class 47s were built at Brush's Falcon Works in Loughborough and at British Railways' Crewe Works between 1962 and 1968, which made them the most numerous class of British mainline diesel locomotive. They were fitted with the Sulzer 12LDA28C twin-bank twelve-cylinder unit producing though this was later derated to to improve reliabilityand have been used on both passenger and freight trains on Britain's railways for over 55 years. Despite the introduction of more modern types of traction, a significant number are still in use, both on the mainline and on heritage railways. , 78 locomotives still exist as Class 47s, including 31 which retain mainline running certificates and a further 31 which have been preserved. 33 locomotives were converted to Class 57s between 1998 and 2004. Origins The Class 47 history begins in the early 1960s with the stated aim of the British Transport Commission (BTC) to remove steam locomotives from British Rail by a target date of 1968. It therefore required a large build of Type 4 but with an axle load of no more than . The BTC was not convinced that the future of diesel traction lay down the hydraulic transmission path of the Western Region, and concentrated on diesel-electric designs. Initially, the BTC invited tenders to build 100 locomotives to the new specification. The following responses were received: A consortium of the Birmingham Railway Carriage and Wagon Company, Associated Electrical Industries and Sulzer offered a production version of their D0260 Lion prototype under construction at the time, with both steam and electric heating Brush Traction offered three options; a production version of their twin-engined D0280 Falcon prototype under construction, or a single engine design using either a Sulzer engine or the English Electric 16CSVT, including steam heating with an option for electric heating English Electric offered a design based on the eventual DP2 prototype North British Locomotive Company offered a Sulzer-powered design with steam and electric heating Of these bids, the BRCW/AEI/Sulzer bid was the preferred option, but before the prototypes could be assessed, the need to build a large number of locomotives quickly was deemed paramount, and the BTC decided on a new approach: it decided to cancel the final order of twenty Class 46 locomotives and