The British Rail Class 37 is a diesel-electric locomotive. Also known as the English Electric Type 3, the class was ordered as part of the British Rail modernisation plan. They were numbered in two series, D6600–D6608 and D6700–D6999. The Class 37 became a familiar sight on many parts of the British Rail network, in particular forming the main motive power for InterCity services in East Anglia and within Scotland. They also performed well on secondary and inter-regional services for many years. Many are still in use today on freight, maintenance, and empty stock movement duties. The Class 37s are known to some railway enthusiasts as "tractors", a nickname given due to the similarities between the sound of the Class 37's engine and that of a tractor. Description Background As part of the large scale dieselisation brought about by the British Rail modernisation plan a need was identified for a number of type 3 locomotives of power output to . English Electric had already been successful with orders for type 1 and type 4 diesels, and had produced locomotives of similar power for railways in East Africa. A design based on the exported locomotives was put forward and accepted. The design was for a general purpose locomotive and initially found service in British Rail's Eastern Region. Building There was no prototype. British Railways placed an order for forty-two Class 37 locomotives in January 1959. The first was delivered in November 1960; it entered service on 2 December. BR had ordered further Class 37s before the last of the original batch had been completed in mid-1962. The final locomotive was delivered to the Western Region on 9 November 1965. English Electric split the construction between Vulcan Foundry at Newton-le-Willows, and Robert Stephenson and Hawthorns of Darlington. The 309 locomotives produced in total were originally numbered in the range D6700–D6999 and D6600–D6608. The bodywork bears a strong family resemblance to other English Electric designs such as the Class 40 and Class 23 'Baby Deltic'. Seven orders were placed with English Electric, as follows: Duties The class was designed for both passenger and freight work. Many of the original locomotives were fitted with boilers for steam heating. D6700-6754 were fitted with boilers from new, along with D6758, D6775, D6781–D6818, D6875–D6892. D6960-6968 received boilers from D6701-6709 during 1967/68. 37247 was fitted with a boiler in 1977. With the withdrawal of many Type 2 and Type