Also mounted on the backhead are the locomotive's speed indicator; the regulator valve used by the fireman to reduce dangerous boiler pressures; the duplex safety valve, which protects the crews from potential explosions; the reverse lever or screw, used by the engineer to change the reversing link; the mechanical lubricator controls which freed the fireman from clinging to the swaying running boards as he oiled moving parts; and the injector valves which the fireman uses to control the water levels in the boiler and on the crown sheet.