You asked for an update, so here it is:
Great Smoky Mountain Railroad was chosen to investigate further. We've spent a significant amount of work speccing out the ideal requirements. We went through google streetmaps in detail, worked out a list of feature assets, industries, types of sessions, branch-line options etc. We contacted local rail tourism operators, looked into appropriate locos and rolling stock.
Ultimately as we calculated the amount of work required to do this fully in-house to the standards we're aiming for, even at this reduced scale it did not then fit within our "achievable in the time frame" goal.
However, all is not lost!
Instead, we're now planning to introduce this route (along with our research and goals for it) as a feature project for the upcoming "Multiplayer Surveyor" feature.
Yes, I know that also has been upcoming for a while, but we've been having regular in-house tests and made extensive progress on supporting all the streaming required to make operations smooth and in sync. We're about to get our first group of external testers on board "real soon", so it is definitely getting closer!