It is important to know what tiles are. A floor is a floor. A wall is a wall. An up ramp is a floor. A down ramp is a hole in the ground.
You can't carve a hole in the ground or a wall. What you carve are the floors.
I would also add: the tracks are carved in half-tiles that go from the tile center to one of tile's edges. This is a bit confusing since when you designate, you select whole tiles instead of halves.
When you designate 3 floor tiles in a straight E-W line, the resulting track will occupy 4 half-tiles: the leftmost designated tile will only have one half going from its center to the eastern edge, the middle tile will have two halves and the rightmost will have one half between its western edge and its center.
With ramps it comes with a twist - to have both halves of the ramp covered, you need to designate the neighboring tiles
on the level on which the ramp is. This is also confusing, since these neighboring tiles might be walls or downward ramps which means that this designation will not result in any tracks carved on these tiles - you only need to include them as the clue to the carver so that both halves of the ramp will get carved.