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. Release valves are provided on the brake cylinders; when operated, usually by manually pulling a cord near the cylinder, air is admitted to the upper part of the brake cylinder on that vehicle. This is necessary to release the brake on a vehicle that has been uncoupled from a train and now requires to be moved without having a brake connection to another locomotive, for example if it is to be shunted. In the UK the pre-nationalisation railway companies standardised around systems operating on a vacuum of , with the exception of the Great Western Railway, which used . An absolute vacuum is about , depending on atmospheric conditions. This difference in standards could cause problems on long-distance cross-country services when a GWR locomotive was replaced with another company's engine, as the new engine's large ejector would sometimes not be able to fully release the brakes on the train. In this case the release valves on each vehicle in the train would have to be released by hand, before the brake was recharged at 21 inches. This time-consuming process was frequently seen at large GWR stations such as Bristol Temple Meads. The provision of a train pipe running throughout the train enabled the automatic vacuum brake to