I'm wondering if any pc can handle a route that long? I've done Lithgow to Bathurst and that's 180 boards alone. As for the tunnels I did number 10 as part of the Zig-zag, with no problems and that is a slightly "S" shaped tunnel and one of the longest from memory? I assume you have the dig holes in place, and the tunnel must be one piece for that to work. Adjusting their spline points is no different to laying track, and from memory I laid the track first at the correct grade so the tunnel could be laid and adjusted to match.
I have a medium speed, aka older PC, and I run heavily forested routes of well over 190 miles / 305 km and there are few issues with the performance. What I run into is AI getting stupid on the route that long, and my interest faltering after an hour or so of driving.
Now this route had little in the way of me doing any building and instead it was a hack and sack merger and acquisition of many routes by famous builders. Its length allows me to run long US-type freights and service the multiple industries along the route both active and inactive. On first glance, it's a wonderful experience running a route this long, but as time goes on I have found that it's not in my genes any longer.
Building such a route this size from scratch, is not for the faint-hearted, especially if one wants the quality and level of detail that one expects from his or herself. Seriously anyone can slap together a 300 km route easily with little attention to detail and haphazardly laid track, but is this the quality you want. The long route requires just as much attention to detail as any other route does. The problem with the long route is the amount of work placing objects. Sure somethings can be copied and pasted such as swaths of forests, but what about those places with towns and other detailed places? This is where things get bogged down as we hunt for details, and on a route of this size it doesn't happen just once.
Once the route is completed, setting up the session is not a short task either. On my merger and acquisition route, the operating session took two weeks to get things operating mostly how I liked it. Placing the multitude of loose consists around the route was painful at best, and that took most of the time because what's the point of having a huge route without consists in yards, engines in terminals, and other interesting things. The next step was setting up the portals and schedules. I did use the Schedule Library and its related Copy Commands from driver command for this, but still this was many hours of work. Once setup, there was the troubleshooting, made easy by the schedule library, but still it was a very lengthy process.
Once all is setup, the AI start off fine, but as time goes on they become stupid. The performance is fine, meaning the frame rates are okay, but the AI begin forgetting commands, skipping, backing up, running signals, and so on. Then there's the issue with the threads getting stuck, a known bug and a big one too, where the AI just stop and no to anything. Pressing pause works for that at first, but not afterwards so it's a one shot-deal. The only way to get around this is to save the session, quit completely, and come back in. This is riddled with the usual mess of AI crashing, signals not lighting, and industries getting messed up. Once this starts, it's a downwards spiral to no return.
With that said, I'm not saying don't go for the super route, but it's something that you might find boring, frustrating, and just too big to chew on.