I just purchased "Ohio Steel 2" via STEAM. When I tried to run a scenario, the simulator could not load the route due to missing files. Rather than fiddle around trying to fix the problem, I requested a refund. Q - Are there undocumented dependencies on assets from other routes?
For a start, make sure that all of these are installed in your system: https://steamdb.info/sub/13229/apps/ You don't actually need the four loco packs I'm sure. These are all supplied with OS2 so you should have them, however you might have some of those uninstalled. Occasionally things seem to go south and one needs to reinstall new DLC, sounds arcane but does the trick.