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 Inter-City services in East Anglia and within Scotland. They also performed well on secondary and inter-regional services for many years. The Class 37s are known to some railway enthusiasts as "Tractors", a nickname due to the agricultural sound of the diesel engine of the locomotive.
This designation covered all 309 locomotives as built, but with such a large number of locomotives and with two companies involved in the building, there were several differences within this sub-class alone. The most visible external difference was that the first 119 locos had a "split" headcode box; for these locos the four digit Train reporting number was shown in two square boxes containing two digits and separated by a pair of doors designed to allow the train crew to be exchanged while in motion. Later locomotives had a single centrally placed headcode box, and also had the horns mounted on the roof rather than built into the nose of the locomotive. This difference was the reason for the double change in numbers when implementing the TOPS scheme described earlier.