The British Rail Class 37 is a diesel-electric locomotive. Also known as the English Electric Type 3, and by some railway enthusiasts as 'tractors' due to their agricultural sound, 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 was designed for both passenger and freight work. Many of the original locomotives were fitted with boilers for steam heating. With the withdrawal of many Type 2 and Type 3 locomotives in the 1980s the 37s were selected as the standard Type 3 and many of the fleet were given a heavy overhaul to prolong their life into the 1990s and beyond. Some were fitted with electrical train heating (ETH) equipment in the 1980s to become the 37/4 sub-class, initially for use on the West Highland Line, the Welsh Marches line and South Wales - Bristol area services and Far North Lines but later seeing use in north/mid Wales and occasionally the West Country. In 2010, they were used on passenger services on the Cumbrian Coast line and Wherry lines. Class 37 locomotives have proved to be very popular, with many examples saved for preservation on heritage railways as well by enthusiast groups. A staple of the Hornby line, who produced the first version of the BR Class 37 in Dublo back in 1965.