The class 37, known as the English Electric Type 3 until TOPS numbering was introduced in 1968, is one of Britain's best-known diesel locos. Their use began in 1960 with the first production batch of 42 locomotives and by 1965, the total had increased to 309. With the classification into power class 3 from 1,500 to 1,999 hp, it had a low weight for a locomotive with six axles, which made it suitable for use on most branch lines.