horizontal bar behind the cylinder. Piston rodConnects the piston to the cross-head. PistonProduces the motion for the locomotive from expansion of the steam. Driven backward and forward within the cylinder by steam delivered alternately, in front and behind, by the valve. CylinderChamber that receives steam from the steam pipe. ValveControls the supply of steam to the cylinders. The valve gear, actuated by connection to the driving wheels, ensures that steam is delivered to the piston with precision. Types are slide valves, piston valves or poppet valves. Valve chest / Steam chestValve chamber next to the cylinder (24) containing passageways to distribute steam to the cylinders. FireboxFurnace chamber built into the boiler, which produces steam in surrounding water. Various combustible materials can be used as fuel; the most common are coal and oil but in earlier times coke and/or wood were used. Boiler tubes and fluesCarry hot gases from the front of the firebox to the front of the boiler, producing steam from the surrounding water. Flues are larger in diameter than tubes because they contain superheater units. BoilerHorizontal tubular vessel, strong enough to contain high-pressure steam in a harsh working environment; closed at either end by the firebox and tube plate. Usually well filled with water but with space for steam – produced by heat from the firebox and boiler tubes – to be above the water surface. Superheater tubesPass steam back through the boiler to dry and superheat it for greater efficiency. See Superheater (32). Throttle valve Regulator valve Controlled by the Throttle Lever / Regulator (8), regulates the amount of steam delivered to the cylinders, which is one of two ways to vary power of the engine (throttle governing). For the other method, see Throttle lever (8). SuperheaterProvides additional heat – as much as hotter – to steam that has been generated in the boiler by sending it back through superheat tubes located in the boiler tubes (28), thus increasing engine efficiency and power. See also Superheater tubes (30). Smokestack Chimney, Funnel Vertical pipe on top of and inside the smokebox that ejects the exhaust (smoke and steam) above the locomotive. HeadlightLight on the front of the smoke box to illuminate track ahead and warn of the approach of the locomotive. Brake hoseHose for conveying force to train brakes by a differential in air pressure. Contains either high-pressure compressed air or air at lower than atmospheric pressure (vacuum),