The largest type among the Heisler locomotives was the 90-12-40. This meant a service weight of 90 short tons, twelve wheels and a wheel diameter of 40 inches. With this, an additional tender was stored on a third, two-axle bogie, which was also connected to the drive shaft. Although Charles L. Heisler's patent originally included four-cylinder locomotives, the largest locomotives were also built with two cylinders and an increased boiler pressure of 200psi. The maximum train weight was around 600 tons on a gradient of two percent and 225 tons at five percent. In theory, up to 4,000 tons were possible on the flat, but these geared locomotives were not intended for the flat country.

A surviving example of the large design is the standard gauge number 6 of the Cass Scenic Railroad. The latter operates an eleven mile long, very hilly line in the Allegheny Mountains in West Virginia, which was built in 1901. On the Cass Scenic Railroad, the Heisler, built in 1929, is the only one of its kind, while the other engines consist of a Climax and otherwise only Shays. Today, like the rest of the fleet, it is still owned by the state of West Virginia, but is now used in Durbin by the operating company Durbin and Greenbrier Valley Railroad. The locomotive was originally built for the Meadow River Lumber Company, which at the time operated one of the largest sawmills in the world.