the british rail (br) class 50 is a diesel locomotive built from 1967–68 by english electric at their vulcan foundry works in newton-le-willows. fifty of these locomotives were built to haul express passenger trains on the, then non-electrified, section of the west coast main line between crewe, carlisle and scotland. they were originally hired from english electric leasings, not being purchased outright by br until around 1973. under the pre-1968 classification system these locomotives were known as the english electric type 4. the class were affectionately nicknamed "hoovers" by rail enthusiasts because of their distinctive engine sound, caused by the centrifugal air filters originally fitted. these proved unreliable, and were later removed, but the "hoover" nickname stuck.