The Heisler locomotive was the last on the scene of the three major types of geared locomotive, Charles L. Heisler receiving a patent for the design in 1892 following the construction of a prototype in 1891. Somewhat similar to a Climax locomotive, Heisler's design featured two cylinders canted inwards at a 45 degree angle to form a 'vee-twin' arrangement, driving a longitudinal driveshaft that drove only the outside axle on each powered truck. The Heisler was the fastest of the geared steam locomotive designs, and yet was still claimed by its manufacturer to have the same low speed hauling ability.
The first Heislers were built by the Dunkirk Engineering Company of Dunkirk, New York, who at the time produced their own different design of geared locomotive of which the Heisler could be considered an improvement. They did not adopt the Heisler design, but in 1894 the Stearns Manufacturing Company of Erie, Pennsylvania started to produce Heislers, which it did until 1904. Reorganised as the Heisler Locomotive Works in 1907, it produced locomotives to the Heisler design until 1941.
Heislers were produced in both two and three truck variants in sizes ranging from 17 tons to 95 tons. Approximately 625 were produced, of which around 35 still exist. About eight of these are currently operational.