Miles - Agree except what would cause the loss of momentum of the jumper in the train that is traveling at constant velocity? Assume that everything is linear, no earth curvature or relativistic effects (cheating yes but handy).

The only forces acting on the jumper while in the air aero forces which are probably at a steady state condition. Gravity is constant down. I can't come up with anything that would give an acceleration to the jumper relative to the train in the longitudinal direction.

Ben