T-man, I think you mean 18" radius curves (36" circle) and I think the next size up may be 22" (44" circle)
Also, keep in mind that if running the lionchief trains with the remote controls then the loops may need to be separate.
Lastly, the track layout in your link appears to be on a 4.5' x 8' platform if I read it correctly.
The inner track is 027 and the outer is O gage.T-MAN nice lay out. I want to do the same thing you have. The 3 track. One outside oval and than 2 inner ovals. I just have to find out what curves would work with fast track. I already have 36” curves. 4x8 is my platform. Thanks