Locomotive servomotor, 19th century. Illustration of a servomotor mechanism used on a steam-powered locomotive. This term is sometimes used to refer to a locomotive's reversing gear. Artwork from 'Chaudieres et Machines a Vapeur' (1911) by French civil engineer Max de Nansouty (1854-1913), part of the 'Les merveilles de la science' series of 1867-1891 by Louis Figuier.