André Xavier Chapelon was born in Saint-Paul-en-Cornillon, Loire, France on 26 October 1892. According to family relatives, his great-grandfather James Jackson immigrated to France from England in 1812, one of many who came to France in the 19th Century to teach steel production methods. He achieved a distinction in mathematics and science, and served as an artillery officer during World War I before returning to the École centrale Paris in 1919, from which he graduated as Ingénieur des Arts et Manufactures in 1921. He joined the Chemins de fer de Paris à Lyon et à la Méditerranée (PLM) as a probationer in the Rolling Stock and Motive Power section at Lyon-Mouche depot. However, foreseeing poor prospects, he left in 1924 and joined the Société Industrielle des Telephones, soon becoming as assistant manager. In 1925, he joined the Chemin de Fer de Paris à Orléans (PO) and, along with Finnish Engineer Kyösti Kylälä, jointly designed the Kylchap exhaust system. While his principles met with scepticism, No. 3566, the first locomotive rebuilt to Chapelon's design, was an outstanding success, and from 1929 to 1936 several other locomotives were rebuilt to Chapelon's designs.