Every time you add a self created scenario to Main Spessart Bahn and try to play it the game crashes. So you are not able to create and play scenarios there!
On my Xbox One X I don't get a crash when starting a custom scenario. I'm also playing only in HD mode (not 4K). I only put two services in it, though. Was wondering why I couldn't move the train, then realised I was on a non-electrified station near the docks.