Many of the earlier classes of diesel locomotive used on British Railways (and electric locos up to and including the Class 86) were fitted with dual systems to enable full usage of BR's rolling stock inherited from the private companies which had different systems depending on which company the stock originated from. Air brakes need a tap to seal the hose at the ends of the train. If these taps are incorrectly closed, a loss of brake force may occur, leading to a dangerous runaway. With vacuum brakes, the end of the hose can be plugged into a stopper which seals the hose by suction. It is much harder to block the hose pipe compared to air brakes. Vacuum brakes can be operated in a twin pipe mode to speed up applications and release. Twin pipe vacuum systems were standard on the 1st generation British Rail Diesel Multiple Units which replaced steam locomotive hauled passenger trains on many branch and secondary lines in the 1960s. The second "high Vacuum " pipe and associated reservoirs and valves were used as a means to increase the speed of the release of the brake. The vacuum exhausters on these units were mechanically driven by the engine; as the engine would normally only be idling when a brake release was required, release would have been extremely slow if the conventional single-pipe system had been employed. This problem did not occur on BR's diesel locomotives, as their exhausters were electrically driven, and so could be run at high speed to release the brake irrespective of engine speed. Today's largest operators of trains equipped with vacuum brakes are the Indian Railways and Spoornet (South Africa), however there are also trains with air brakes and dual brakes in use. South African Railways (Spoornet) operates more than 1 000 electric multiple unit cars, which are fitted with air compressed brakes. The electro-vacuum system uses a train pipe and basic automatic vacuum brake system, with the addition of electrically-controlled application and release valves in each vehicle. The application and release valves greatly increase the rate of train pipe vacuum destruction and creation. This, in turn, greatly increases the speed of brake application and release. The performance of electro-vacuum brakes on SAR EMUs is equivalent to electro-pneumatic braked EMUs of a similar age. Other African railways are believed to continue to use the vacuum brake. Other operators of vacuum brakes are narrow-gauge railways in Europe, the largest of which is the Rhaetian Railway. Vacuum brakes have been entirely superseded on the National Rail system in the UK (with the British Rail Class 121 "bubble cars" being the last mainline trains to have vacuum brakes-they finished service in 2017), although they are still in use on most standard gauge heritage railways.