@jonnyspeed I downloaded your AnyRail file and hope to take a look at it soon. I've been on a helix kick myself lately, and would be really interested in seeing the 3D version!
You have an opportunity to spend quality time with your son. That's the most important consideration. How nostalgic is he about the RailKing? Maybe you could convince him to sell it and go over to "the dark side" (2-Rail) I know it's theoretically possible to run battery-operated 2-rail on hi-rail track, but the smaller flanges may prevent your locos from reliably traversing the switches. @AGHRMatt may have more insight. Most diesels are easy to convert and then convert back; steam could be difficult or impossible unless you're a machinist.
I would have a real heart-to-heart conversation with your son, and whichever way he's leaning, that's the way I would go. Probably in a few years he'll be out of the house, away at college, girlfriend, etc. So you'll have the rest of your life to do what you want. The next few years are precious, so if I were you, I would do whatever I could to keep him close!
@prrjim I would love to hear more about your automated car routing system (and different methods to generate switchlists, etc., in general.) I hope to incorporate this type of operation into my own layout. If Jon doesn't mind you could post it here, or maybe start another thread on this subject.
Once again I'm amazed, inspired, and touched to read about other people's endeavors in the Layout Design forum!!