Can you also be on top of it? If you're "in" the entrance, how do you go the the hill above it? Or is that even possible? Can a unit adjacent to the entrance but on the level above fire into it? Advance into it?
If I remember correctly, the rule only mentions hexs D7 and F7. Note that both of these are Level 2 Hexes, so one is not "in" the entrance as you are assuming. (Or to state it another way, there aren't two possible positions within each hex.) So, of course, one can move (or advance) normally from either hex to E7 or E8.
The SSR just adds the ability to advance directly from D7 to F7 (or vice versa) and to move directly from D7 to F7 (or vice versa) at a cost of 1 MP. (There is also the LOS ability, but that isn't germane to the movement question being asked.)
Edit - Just after I posted, I thought this might be a different (but clearer way) to explain the above. Consider a squad moving from G7 to F7 (along the railway). The movement cost to do so is 1 MP (for entering a hex along a railway) + 1 MP (for moving uphill). The unit at the end of its movement is now in a Level 2 Building in Hex F7.