The discussion refers to this topic:
https://forum.simutrans.com/index.php/topic,19562.0.htmlI also had simmilar issues quite a lot. Currently, when changing a line from mirror schedule operation to explicitly set return schedule operation, it will require the player to check every single vehicle in that line for the reverse route option and unset it if it was set.
Additionally, one somethimes accidently checks that reverse route checkbox when opening the vehicle stats.
Such a mistakenly in reverse route mode operating vehicle is quite hard to find as there won't be any hint or message about this apart from plyers noting "your vehicle seems to do pretty strange stuff, please check your ways and schedules"
I had the following thoughts: If one does not use mirror schedule nor alternate directions, there should not be a reason to reverse the route of a vehicle.
So the suggestion is automatically unmirroring all vehicles of a line when none of these two options is checked and disallowing the reverse route checkbox for vehicles in that line.
If there is a rare case I did not imagine, where mirroring single vehicles without any of these two options makes sense, we should introduce an explicit "allow reversed route" checkbox to the line, which will allow vehicles of that line to use the reverse route checkbox.