The first Electric Type 3 diesel locomotives were ordered by British rail in 1959, a total of 309 of these diesel-electric locomotives were built and powered by a 12 cylinder English Elictric 12CSVT power unit. The construction took place at the works of Vulcan Foundry and Hawthorn Leslie Darlington. Many of the components used on the class were shared with the class 40's and 55's, one of the main parts used was the bogie which was fitted to 50's and the 55's.The first class 37 entered service in December 1960 at Stratford shed (3oA) and was put straight into passenger duty on Liverpool Street to Norwich and Clacton Services, plus mixed traffic duties throughout the rail network. As part of refurbishment programme a numder of changes took place, including lower gearing on some locos to increase the traction effort. Other locos were fitted with extra ballast and some had new more powerfull motors fitted thus creating a number of sub-classes.