Our Cass train was hauled by a Shay and a Heisler. Both are geared locomotives, designed for use with steep grades and sharp curves (which abound at Cass, a former lumber line). The Shays are the big attraction at Cass -- it has the largest collection of operating Shays in the world.
The locos are always on the downhill end of the train (for safety reasons), so they basically push the cars up Bald Mountain, then pull them down again. There are two locos going up (for speed and power), and just one coming down (when gravity helps!). The brakes on the cars are used during the descent to keep the train under careful control, especially on the switchbacks where the locomotive is at the uphill end.