The Heisler is the geared steam locomotive design that uses a V-twin-style engine.
A typical Heisler has two steam cylinders canted inward at about 45 degrees, forming a V beneath the boiler. The cylinders drive a longitudinal shaft running along the locomotive’s centerline. Bevel gears then transmit power to the axles of the powered trucks.
Charles L. Heisler patented the design in 1892 after building a prototype the previous year. Heislers became popular in logging and industrial service because their geared drive delivered high tractive effort at low speeds and allowed them to negotiate rough, lightly built track.
The answer is often confused with the Climax and Shay, the other two major North American geared-steam designs. A Shay typically used cylinders mounted on one side of an offset boiler, while a Climax used a different cylinder and drive arrangement. Heisler locomotives were usually built with two or three trucks, and some later examples were converted to internal-combustion power.