I have the same problem with splines (both track and roads) on a large route I had been working on. I have posted on this problem in the past.
I tried deleting vacant baseboards and then save/exit the route/reload the now much smaller route. No luck. That route is corrupted apparently.
But all is not lost. With Surveyor 2.0, you can select fairly large areas from the corrupt route, copy to the scrapbook, and then paste to a new route. I have done this for fairly large areas (10 x 10 baseboards for example). I also upgraded my RAM from 16GB to 32GB for under $50 USD, so consider that as well.
Before you do any of this, try this: Exit Trainz, then restart the computer, then reload Trainz, then reload the route. Does this make a difference? I have found sometimes it does, sometimes it doesn't. If it helps, this suggests a memory issue with the newer versions of Trainz.