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. 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.