This steam locomotive is a special kind known as a Shay, and they were designed primarily for the steep grades and tight curves of a mountain logging railroad. There were many types and sizes with this 150 ton model being among the largest, but they all had the common characteristics of having an offset boiler and the driving pistons mounted vertically along one side. The pistons powered a drive line to power all the wheels which were geared, and the entire locomotive sat on one frame with no trailing tender. All of those things can be seen in this action view, and what you are seeing is Victorian style 12 wheel drive! Every wheel is helping to drive the locomotive.
This locomotive belongs to the Cass Scenic Railroad in West Virginia, which has always run nothing but these Shays because their railroad grades are so steep. One short section is 11% which is impossible to climb on typical railroads, even modern ones, but the vintage Shays go up and down on a daily basis without problem. They aren't fast at all, but they are truly mechanical mountain goats. Take a ride up through the steep mountain woods and see for yourself, and end up on top of the mountains at a mile high.
Cass Scenic Railroad State Park consists of the railroad with fully outfitted shops and the restored town of Cass that housed all the railroad and lumber workers. The worker houses are available as vacation rentals and the company store remains, but the huge mill that processed the timber hauled by these mighty little locomotives burned down some years back. For more info see www.cassrailroad.com