You cannot fall up,simple as that.
You can “RUSH” up, however. That train would really be moving by virtue of the gravity which had acted upon it as it went down. And in an ideal frictionless environment, the energy it would have by moving in the middle would be just enough to get it back up to the top at the other end.
You don’t know squat about the physics, pal.
Let me give you an even more obvious example: a roller coaster with a “suicide” drop. It comes down a straight away, hits a U-shaped bottom, and goes back up another straight away. If this lacked friction, yes the coaster would “fall up” all the way to its original level from the bottom. Even with friction it would go back up a goodly ways without additional help from motors. This is not like plopping straight down into a pit and smacking square into a bottom.