The park has a number of steam locomotives they use, all of which are specially designed for power, rather than speed, so that they can climb the steep grades (up to 11%) on the mountain. Another feature of their design is that they work equally well in forward or reverse. That is handy on the switchbacks.
Oddly enough, the engines are attached to the rear of the train, but facing forward. They will push the train up to the first switchback, then pull it (backwards) into the second, then push it again.
A second engine is hooked up, and they take off, running at about 4 miles per hour on average.