There's a decent video out there showing that, if you wanted to be a true psycho and just immediately kill everyone you come across, the game literally cannot be progressed. So they made it so that literally, no matter what you do, Preston will still stay green, and still help you to get the teleporter built to the institution. You cannot destroy every faction, if you wanted, as the institute would remain unreachable. So the railroad, even if you try to kill everyone in it, is still there to help. All they needed to do was make it so that you could still get the plans for the teleporter and built it yourself, if you decided to kill everyone, everywhere. But, no.