lever (US: Johnson Bar) or reversing lever or wheel (UK+) in the cab to the valve gear. Safety valvePressure relief valve to stop the boiler pressure exceeding the operating limit. Generator / Turbo-generator Electrical generator driven by a small steam turbine, for the headlight and other locomotive lighting. Sand domeHolds sand that is directed on to the rail in front of the driving wheels to improve traction, especially in wet or icy conditions or when vegetation is on the line, and on steep gradients. Throttle lever Regulator Sets the opening of the throttle valve / regulator valve, (31) which controls the amount of steam entering the cylinders, hence the speed of the locomotive. It is used in conjunction with the reversing lever to start, to stop, and to control the locomotive's power output. When the regulator/throttle is closed, a vacuum valve (snifting valve) permits air to be drawn through the superheater and cylinders to allow the engine to coast freely. The throttle is not the only control that can limit the locomotive's power output: during steady-state running of most locomotives, the throttle is usually set wide open and the power output is controlled by moving the reversing lever (2) closer to its mid-point ("reducing the cut-off") to limit the amount of steam admitted to the cylinders. Steam domeCollects steam at the top of the boiler (well above the water level) so that it can be fed to the engine via the main steam pipe, or dry pipe, and the regulator/throttle valve. Air pump / Air compressor Westinghouse pump Powered by steam, it compresses air for operating the train air brake system. The Westinghouse air brake system is used world-wide; in Europe two systems that use the same principle are the Kunze-Knorr and Oerlikon systems. It can be a single-stage or, when larger capacity is needed, a two-stage cross-compound compressor. Vacuum brakes, used historically, do not employ compressors; as a result of their relative inefficiency they are no longer in large-scale use. SmokeboxReceives the hot gases that have passed from the firebox through the boiler tubes and, when the throttle/regulator is open, directs them and steam exhausting from the cylinders up the smokestack/chimney, sucking air through the firebed. The smokebox may contain a cinder guard to prevent hot cinders being expelled. registration: there are other constraints (such as a cap on daily views) to freely access this source. Components in the