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.
These first 20 locomotives, originally numbered D5500–D5519, were always easily recognisable as they did not have the headcode box mounted on the roof above the cab, leading to the nickname "Skinheads". They were also nicknamed "Gurglers" from the noise of their engines, and "Toffee Apples" from the shape of the control key which had to be taken from cab to cab when changing ends. These pilot scheme locomotives were non-standard in having Electro-Magnetic Multiple-Working control equipment, and were limited to 80 mph (130 km/h). After being involved in a serious collision D5518 was rebuilt in September 1967 as a standard locomotive, with indicator boxes, and blue star coupling code.
They were allocated to East Anglian sheds throughout their service, ending up allocated to Stratford, their initial shed, and latterly sporting that depot's trademark silver roof. Upon withdrawal four locomotives were converted at Stratford into train pre-heating units. Locomotives 31013, 31002, 31014 and 31008 were renumbered ADB968013 to ADB968016 in the order given; ADB968014 was allocated to Bounds Green depot on the Great Northern main line and ADB968015 was based at Great Yarmouth, while the remaining two were allocated to Stratford.