When a minecart goes over the above end of a slope with a rail under the top block (like seen in the images) (either derection) it, visually, teleports downwards to the rail under the slope for a short moment. The hitbox stays in the correct position. The type of minecart and type of rail does not matter, neither does the derection of the bottom rail relative to the top rail. There is no sign the minecart is on the bottom rail functionally: tnt minecarts doesn't explode when the bottom rail they teleport to is an activated activator rail, and chest minecarts don't put items when there is a hopper under that same rail.