Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power. The valve consists of two piston heads on a common spindle moving inside a steam chest, which is essentially a mini-cylinder located either above or below the main cylinders of the locomotive. Overview In the 19th century, steam locomotives used slide valves to control the flow of steam into and out of the cylinders. In the 20th century, slide valves were gradually superseded by piston valves, particularly in engines using superheated steam. There were two reasons for this: It is difficult to lubricate slide valves adequately in the presence of superheated steam With piston valves, the steam passages can be made shorter. This, particularly following the work of André Chapelon, reduces resistance to the flow of steam and improves efficiency The usual locomotive valve gears such as Stephenson, Walschaerts, and Baker valve gear, can be used with either slide valves or piston valves. Where poppet valves are used, a different gear, such as Caprotti valve gear may be used, though standard gears as mentioned above were used as well, by Chapelon and others. Most piston valves are of the "inside admission" type, where fresh steam is introduced from the boiler via the space between the two piston heads of the valve, and exhaust steam leaves via the space between a piston head and the end of the steam chest. The advantage of this arrangement is that leakage, via the gland which seals the steam chest from the operating rod of the valve gear, is much less of a problem when the gland is subjected to low exhaust pressure rather than full boiler pressure. However, some locomotives, like Bulleid's SR Merchant Navy class, used "outside admission" where the reverse was true, in Bulleid's case because of the unusual chain-driven valve gear arrangement. Examples The Swannington incline winding engine on the Leicester and Swannington Railway, manufactured by The Horsely Coal & Iron Company in 1833, shows a very early use of the piston valve. Piston valves had been used a year or two previously in the horizontal engines manufactured by Taylor & Martineau of London, but did not become general for stationary or locomotive engines until the end of the