Because that is two distinct moves. You may only make one distinct move in the APH. Again, what you're asking is like wanting to move into the ground level of a building and go upstairs. That's two moves even in the same hex.
I believe the long and short of what you're asking is two distinct MF expenditures which is not allowed in the APH. Same as entering a building and then trying to go upstairs.
Just to be clear, as I read it, if a rail car counter is placed in the hex any railroad is a Level 1 obstacle to LOS except on the hexside(s) UNLESS the railroad crosses to another hex with a rail car counter.
I will say this, the example in the rule book is terrible. Put some infantry counters in those hexes instead of letters just placed willy nilly on the side of the hex. That was super lazy.