The automatic brake works by monitoring the speed using a centrifugal governor connected by gears to a large toothed gear mounted on the pinion axle next to the wheel (i.e. it is not the pinion gear). When the set speed is exceeded a lever on the governor hits a lever on a brake valve, and the brakes operate. The automatic brake can only be released after the train has come to a stop, and the driver has to leave the locomotive to reset the system. On the steam locomotives, steam is applied to a small brake cylinder that acts on the driver's side brakes. On the carriages, the automatic brake is an air brake. On the diesel locomotives, a hydraulic cylinder is used.