The British Rail Class 31 diesel locomotives, also known as the Brush Type 2 and originally as Class 30, were built by Brush Traction from 1957-62, numbering 263 locos. The Class 31 entered service in November 1957 and was one of the Pilot Scheme locomotives ordered by British Railways to replace steam traction. The class was originally allocated to the Eastern Region, but gradually became common in both the Western and London Midland regions too.
The first few locos externally had much in common with the original 31/0s as twenty lacked the roof mounted headcode box and fifteen were also limited to 80 mph but were otherwise the same as subsequent locos. The whole sub-class had steam heating boilers fitted, had the Blue Star Electro-Pneumatic multiple-working controls as found on many other BR classes. The Class 31/1s could be found on a variety of secondary and relief passenger duties as well as parcels and freight traffic. While used in East Anglia, with locos allocated to Stratford and March depots, they were found throughout the Eastern Region of BR with Finsbury Park sporting a large allocation along with the depots at Tinsley, Immingham and Thornaby. Locos were also allocated to Bristol Bath Road and Old Oak Common on the Western Region, where they could be found working passenger trains as far west as Barnstaple and Paignton. In the early 1980s Healey Mills and Bescot on the Midland Region also gained an allocation as replacements for Class 25s.
The Class 31/4s, numbered from 31 400 to 31 469, were conversions of 31/1s to which ETH was fitted. They had an ETH index of 66, equivalent to 330 kW, which was sufficient to power trains of up to eleven Mk 3 carriages. This allowed them to pre-heat long trains, whose service run would be worked by a larger locomotive, between depot and terminus, although in actual passenger service loads rarely exceeded four or five carriages. 330 kW accounted for about a third of the total electrical power output. The early conversions tapped off the main generator such that none of the ETH power was available for traction even if the ETH was not being used. The traction power output of some of the 31/4 subclass was therefore limited to a maximum of two-thirds of that of the non-ETH variants, this did not help the performance of an already somewhat underpowered locomotive, and late running of these 31/4-hauled services sometimes happened. Later conversions allowed unused ETH power to be used for traction.
Two locomotives received the modification of through-wiring for Electric Train Heating (ETH), meaning they can be coupled to a train behind another locomotive (with full ETH capabilities), and the front locomotive is still able to heat the train.