I have to say I’m not aware of cases where these locomotives would use their capstan drum to pull themselves up hills, though I can see how that could happen. The cylinders turn a shaft with a gear on it that spins the large gear out front, which is either engaging another gear on the front axle to move the locomotive- the side rod powers the rear axle- or that large gear can be bared over to power the drum/donkey engine. My understanding is the donkey part was typically used to skid logs into a landing where it would be loaded onto flatcars. It’s worth noting the Falk, the Bear Harbor gypsy’s stablemate at Fort Humboldt, is a typical direct drive steam locomotive, its donkey engine is powered by a small third cylinder.

Jeff Moore
Elko, NV