With the introduction of diesel and electric locomotives by British Railways from the early 1950s, this same basic arrangement was carried over. BR's Modernisation Plan of 1955 called for, amongst other things, a long-term aim to switch to air brakes for both passenger and freight stock. The standard Mark 1 coaching stock had been designed and procured before the decision to switch to modern traction and air brakes had been taken, so the majority of the stock was fitted with traditional vacuum brakes. Air-braked goods wagons were introduced steadily from the mid-1960s (starting with the HAA-type hoppers for the new 'merry-go-round' trains) and the Mark 2a coaching stock with air brakes was built from 1967. Diesel and electric locomotives naturally could not use the traditional steam-driven ejector to generate vacuum. Smaller locomotives had exhausters or vacuum pumps driven directly by their prime mover while larger ones had similar machines mounted separately and driven by dedicated electric motors. It was normal practice on mainline locomotives to fit two exhausters (and two air compressors for the air brake system) for redundancy. Just as steam locomotives had a small and large ejector, the diesels and electrics (most of which had braking equipment supplied by either British Westinghouse or Davies and Metcalfe had their brake controls set up to run one exhauster continuously to generate and maintain the vacuum in the system (analogous to the small ejector), with the second one being started when the brake handle was set to its 'Release' position to provide a quicker response (the same function as the large ejector). A switch in the locomotive cab allowed the driver to choose which exhauster would serve each function.