Originally I was thinking to propel a vehicle true the natural movement of the people inside it. This was rather hard, the engines get much to large compared to the energy gains. I assume A system with oil or air pressure would fit the task best. But searching for ways to do that I kept running into much better sources outside the vehicle. They are almost to obvious, things like the wind and going down hill. The main question became: Why use windmills to propel a train? Have we gone crazy?? ah?? I drew up a quick and dirty sketch here.
gabydewilde - Sailtrain