The 64 thousand dollar question. The route does come first, purely for the reason we need somewhere to run our trains. Somewhere along the line
the two disciplines come together, followed by a third, the activity/scenario covering aspect. The end-user is at the end of the chain, so to speak.
I have been involved in a complex project where it wasn't possible to predict whether :-
1) An accurate depiction of the line which included 3 miles of 3rd rail incline, could be done,
2) would the physics of (MSTS) allow the successful running of trains and their unique locomotives under prototypical operations (yes it did).
The route was required first to test the physics of the locomotives, etc. In fact both were developed concurrently, by mutual agreement.
It was a cooperative effort, which is how such routes are developed and delivered to the community. US and NZR narrow gauge operations are another case in point.
You need both the chicken and the egg, therefore the question is a case of Occam's razor.
I voted for the first proposition.