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 the total had increased to 309 by 1965. 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.

The heart of the locomotives was the well-known diesel from English Electric, which was used here in the variant with twelve cylinders, turbocharger and intercooler. The distinctive exhaust sound led to the nickname “tractor” and is probably one of the reasons for the great popularity of the locomotives, some of which are still in use today, among enthusiasts. With a maximum speed of 90 mph, they were equally suitable for goods and passenger trains. For this reason, many locomotives were equipped with a steam boiler, the water tank of which was housed together with the fuel tank between the bogies. 

The boxes for displaying the four-digit train number, which were once found on almost all British locomotives, can be found on the class 37 in two distinctive designs. The earlier design, with two separate boxes, was mostly found in their initial service area of northern and eastern England and was referred to as a “split (headcode) box”. The later built machines only used a center box, more commonly seen in the South West and Wales. With the later conversions, the displays were removed and their lighting used as a headlight.

37 405 and 37 419 operated by Direct Rail Services in sandwich operation with only two passenger cars in December 2016 on the East Suffolk Line

Jeremy Segrott

With the introduction of more powerful locos, the class 37 became more widely distributed around the British Isles in line with current demand. In the Scottish Highlands, for example, they sometimes served almost all locomotive-hauled traffic, while in South Wales they were used to pull the heaviest coal trains. They were very well suited for these services because they had a high adhesive weight compared to their engine power.

Although British Railways phased out many first generation diesels in the 1980s, they could not do without the class 37 and modernized a large proportion of the stock. With the disappearance of steam-heated passenger cars, many locomotives were equipped with electric train heating, creating the subclass 37/4. Others were given new bogies for use in front of goods trains, which only allowed a top speed of 80 mph and thus increased the tractive effort. Some received additional ballast weights to increase adhesive weight. By eliminating the water tank for the steam heating system, the fuel capacity of many locomotives was almost doubled. With these modifications and various different adjustments to the electrics, the subclasses 37/3, 35/6, 37/6 and 37/7 were created in addition to the 37/4, while those that remained in their original condition were subsequently designated as class 37/0. A total of six locomotives were fitted with Mirrlees or Ruston engines in preparation for the development of new locomotives and listed as class 37/9.

Even after the privatization of the British rail system, a large number of class 37s continued to serve with different operators. Demand for passenger transport fell in the 1990s after a large number of Sprinter family diesel railcars had been completed. However, they were able to hold their own in front of goods trains for a long time, especially on routes with low axle loads, until these routes could be released piece by piece by locomotives such as the class 66. Special tasks include the use of departmental, construction or measurement trains and the transport of nuclear waste by Direct Rail Services. In addition to special trains, class 37 locomotives could sometimes still be seen pulling passenger trains after the turn of the millennium, especially when there was a shortage of DMUs. Between 2015 and 2019, passenger transport on the Wherry Lines in the Norwich region was also handled by DRS in sandwich operation with two of these locomotives and usually three passenger cars in order to still achieve good acceleration.