[Semper Vaporo]

Granted the drawing is quite exagerated, but to make the point...

Build your bridge as in "A" and yes, there will be a horizontal component that will cause it to slide toward the lower side and some sort of support will be needed to counteract that force.

Build your bridge as in "B" and NO, there is NO horiontal component, nor more weight on the lower pier than the higher one. There is NO sideways force if the bottom of the bridge at the support points is parallel to the Earths surface.

 

[Semper Vaporo]

The rail would most definitely move if not anchored somehow. Spikes hold the rail to the ties, the ties IN the ballast and the ballast on the ground and gravity holds them together to create the friction that keeps the rail from moving.

Co-worker of mine, many years ago said he had run off the road the night before. He could not get the car to move so he wedged some plywood under the car to drive up on and get a better footing for getting out of the ditch. His wife was behind trying to push. The plywood shot out from under the back of the car and broke BOTH of her ankles!