Hello,
I think this is a really crippling bug that must be fixed:
I got this when trying to save one of my worlds. It didn’t save the game state, as expected. What’s maybe worse, since I used “Overwrite” it also killed the original savegame, so this world is basically lost completely had there been no auto saves. I tried to reproduce it but could not get consistent results. It probably has something to do with train routes. I never ran into this problem without trains and now that I added some, I started to get this. And during my reproduction attempts I also sometimes got this exception after I added more train routes and trains.
Since saving fails, I can’t give you a savegame, obviously 
But I have one that is very close to when it is generally starting to happen. If you’re interested.