The life of the Class 31 actually began as the Class 30, also known as the Brush Type 2. These locomotives used from October 1957 were six-axle engines with four powered axles, fitted with a Mirlees JVS12T with 1,250 or 1,365 hp. The carrying axles were in the middle of the bogies and had a wheel diameter of 3 ft 3.5 in, while the driven wheels had a diameter of 3 ft 7 in. Since the Mirlees engines did not prove themselves, an engine was equipped with the engine of the class 37 in 1964. Since the full power could not be used with four traction motors, the engine was used without an intercooler with an output of 1,470 hp. Since this combination was significantly better, between 1965 and 1969 all remaining 262 pieces were converted in the same way and were then listed as Class 31.
The first batch still had a top speed of 80 mph and electromagnetic controls for multiple working. Later locomotives received electro-pneumatic control gear and were designated as class 31/1, the first batch was subsequently renumbered to 31/0 to distinguish them. The top speed was soon increased to 90 mph. The boxes for displaying the running numbers were placed above the cab windows and the first batch without these boxes soon became known as the “Skinheads”. Since the locomotives had steam train heating, they were used in front of passenger and freight trains. The operations took place primarily on branch lines or as reinforcement with light trains. Later, 70 locomotives were equipped with electric train heating and designated as Class 31/4. Since these diverted up to 330 kW from the engine power, which was not very high anyway, these locomotives were mostly used to assemble and preheat passenger trains.
Network Rail 31233 in August 2013 with a test train at Shrewsbury
From the moment the DMUs of the Sprinter series took over the light passenger train services in the 1980s, the class 31 was almost exclusively used for freight trains. In most cases, the lines for train heating were removed to prevent the locomotives from being temporarily borrowed out to the passenger train division. After privatization, the locomotives came to many different operators. They are used there, for example, to provide additional power for passenger trains. For this purpose, the lines for the train heating were routed through some locomotives without the power having to be diverted from the locomotives themselves. Other examples were used in top-and-tail operation, i.e. as a passenger train with one locomotive each at the front and rear to replace unavailable railcars. Such operations could and can be observed with various older diesel locomotives since privatization. Network Rail, the operator of the UK rail network, uses the Class 31 for test trains across the network. These can be recognized by the completely yellow paintwork on the locomotive and wagons. About 26 examples were bought for operation on museum railways.