This is something that really bugs me and I'm sure I'm not the only one. Lets say I download a GWR class 387 to use for a scenario. I don't want it appearing on random Gatwick Express services on timetable mode. Surely such an easy thing to implement, an option for each livery, "Do you want it to appear on timetable mode or not?". Small quality of life update to improve the game.
This is the exact reason I no longer use a single downloaded Livery. They just randomly pop up all over the place. Until this is implemented, livery designer might as well not exist for me.