Khandro: // are you saying it will hit the same spot on the floor? //
No, it would hit a different spot, but not because of a loss of forward momentum. It's because of the curvature of the earth. While stationary, the train is rotating with the earth, hence floor and ceiling have different velocities, as mibn2cweus said. Whilst moving the train has some additional rotation (or less if it's moving against the earth's rotation), hence the floor-ceiling velocity differential would be different and the landing spot would be different. If we lived on a flat, non-rotating earth, your dropped ball bearing *would* land on the same spot irrespective of the velocity of the train — only acceleration would affect where it landed, not velocity.