Tyler,
I asked Phil Reader about that once, and while the rail could handle a K-37, the radii are too tight. Phil said he would have loved to get the Ashby's #40 out there and give it a go, though he doubted even it would make it all the way around.
A 4-6-0 like the SPNG #9 (which will probably never leave Laws) would probably do fine. But even with a swing bolster a 4 wheel lead truck really adds to the minimum radii an engine can handle. RGS 20 or a T-12 definatly would't make it around with their longer wheel bases.
A 2-6-2, like #12 over at the current GTL operation, would probably be the biggest engine practical.
Cheers,
Curtis F.