Interesting! My use case happens when I have a running line and a new faster locomotive appears, but I don’t want to change all my trains at once. If I only change one of them, its higher speed is useless, since it will always go below the other slower trains. If I were able to build such overpasses, those would allow the faster train to take advantage of its speed.
I can’t use waypoints (it means two lines: one for faster, one for slower vehicles), besides micromanagement, because frequency in both lines would get spoiled. To keep the original frequency or ever a better one, I have to be able to do it in one line…