Editing Compound steam engine (section)

===Application to railway locomotives===
{{Main|Compound locomotive}}
For railway locomotive applications the main benefit sought from compounding was economy in fuel and water consumption plus high power/weight ratio due to temperature and pressure drop taking place over a longer cycle, this resulting in increased efficiency; additional perceived advantages included more even torque.

While designs for compound locomotives may date as far back as [[James Samuel]]'s 1856 patent for a "continuous expansion locomotive",<ref>{{Citation
  |title=Compound Engines facsimile reprint
  |publisher=Scholarly Publishing Office, University of Michigan Library    |location=Ann Arbor, MI
  |year=2005
  |pages=16;17
  |isbn=1-4255-0657-7
}}</ref> the practical history of railway compounding begins with [[Anatole Mallet]]'s designs in the 1870s. [[Mallet locomotive]]s were operated in the United States up to the end of mainline steam by the [[Norfolk and Western Railway]]. The designs of [[Alfred George de Glehn]] in France also saw significant use, especially in the rebuilds of [[André Chapelon]]. A wide variety of compound designs were tried around 1900, but most were short-lived in popularity, due to their complexity and maintenance liability. In the 20th century the [[superheater]] was widely adopted, and the vast majority of steam locomotives were simple-expansion (with some compound locomotives converted to simple). It was realised by engineers that locomotives at steady speed were worked most efficiently with a wide-open regulator and early cut-off, the latter being set via the reversing gear. A locomotive operating at very early cut-off of steam (e.g. at 15% of the piston stroke) allows maximum expansion of the steam, with less wasted energy at the end of the stroke. Superheating eliminates the condensation and rapid loss of pressure that would otherwise occur with such expansion.

Large American locomotives used two cross-compound steam-driven air compressors, e.g. the Westinghouse 8 1/2" 150-D,<ref>1941 Locomotive Cyclopedia of American Practice, Eleventh Edition, Simmons-Boardman Publishing Corporation, 30 Church Street, New York p.813</ref> for the train brakes.