Born October 26, 1892 in Saint-Paul en Cornillon, Loire, France, André Chapelon was possibly the greatest steam locomotive designer and engineer since Robert Stephenson laid down the form to be followed by almost every locomotive built since. His greatest contribution to the state of the art was in bringing scientific method to the steam locomotive. Prior to Chapelon, few engineers and designers tried to understand why a certain design worked better than another - they merely worked by trial and error, trying to replicate the attributes of previous locomotives by rule of thumb, by guesswork, and from empirical theories and design rules that had rarely been given adequate testing.
Chapelon believed in using the best scientific knowledge of subjects like thermodynamics and fluid and gas flow available at the time, and in rigorous testing, using the most accurate and complete testing and sensing equipment available, such as high-speed stroboscopic photography to watch steam flow.
Chapelon's locomotives attained efficiencies that pretty much no other steam locomotives have ever reached - in excess of 12% in some cases.
He was cursed, though, that despite his abilities and track record, he never got to design a class of brand-new locomotives that actually got built. He was continually stymied by railway management and politicians, and often his superbly performing locomotives were treated as embarassments by his superiors - because they showed up the poor performance of the officially approved locomotives!
He was a major proponent of the compound locomotive; his other major work included optimising the steam circuit, including improving the steam flow by widening steam passages and paths, improving the flow through valve gear, and improved exhaust systems such as his Kylchap exhaust. Chapelon realised that to produce an efficient, powerful locomotive that every aspect of it had to be improved and dealt with scientifically. He did work on locomotive behaviour at speed, on the riding properties of the steel wheel on steel rail; his knowledge was put to use much later on the French TGV high speed trains.
The only locomotives he designed for use outside France were some metre gauge 2-8-4 and 4-8-4 locomotives for GELSA of Brazil. They were highly advanced locomotives with many modern American appliances as well as Chapelon's innovations. André Chapelon was much an admirer of American railroad innovation, even though his work tended to be ignored in the USA.
Chapelon's work lives on in the work of L.D. Porta of Argentina, possibly the greatest engineer still living in the steam locomotive field, and others.