ErstO, the reason why this is such a problem is because it is not only happening on the route you are editing. I have had it happen on totally unrelated routes. Yes, the entire rail network becomes interconnected after a time, but when I edit the tracks on Line 1 there is no reason why a train on Line 5 that does not run over that section of track should reverse.
And it’s not just trains that do that. It’s also lines that change direction on a dual track. It makes it hard to keep track of what is going on and it has already cost me a lot of money. And as I said before: it doesn’t really matter when you have become succesfull and have millions to waste. But when you are still in a buildup fase it is troublesome.