The model runs on compressed air that is fed at about 15 PSI to a small stainless steel receiver in the engine's firebox. Lines to the two cylinders are wood, as are the cylinders, and pistons themselves. The pistons are even fitted with wooden piston rings!
The craftsman says of the model:
"I had to use as close tolerances as possible as compressed air does not expand like steam, but also taking into consideration humidity and how much the wood would move. Two air lines run from the cabin valves out to the whistle and one to the valves. The whistle blows at about 2-3 lbs of air and the engine runs really well at about 12-15 lbs of air and stops at 8-9 lbs. It worked on the first attempt, so I stopped and had a beer."
Here is a nice article on the Harold Manwaring and the wooden steam locomotive. Also be sure to check out this video clip showing the wooden locomotive in operation.