American inventor of the so-called Shay type of geared steam locomotive, widely used in the Americas, Australia, and East Asia on logging and mining railroads and in other circumstances requiring relatively small locomotives to move heavy trains at low speeds over rough terrain.

Previously a physician in Ohio and a Union Army engineer in the American Civil War (1861–65), Shay was the proprietor of a sawmill in Wexford county, Mich., when he devised his locomotive. The first Shay was built at Cadillac, Mich., about 1877; the last, at Lima, Ohio, in 1945. Beginning with Shay contracts, the Lima Machine Works (afterward Lima Locomotive Works) eventually became one of the world's most important constructors of conventional steam locomotives.