I though about methods of using gravity as the energy source and came up with tunnel-slides. Huge tunnels that act as a slide at the start, and then momentum pushes you up the other side. would be good for continent-crossing.<br>Clearly impractical now, but could eventually be plausible given improvements in technology,(reply>), You would need some source of propulsion besides just gravity, otherwise each time you go up, you would not go up as high as the starting position.   There would need to be some sort of booster at the end of the uphill.