I've got a section of track on my layout with a 15" radius. My Spectrum 0-6-0T handles it well, but my 3-truck Shay derails. I think a 18" radius would be suitable for the Shay. My Rivarossi 2-truck Heisler handles it like a champ, though. The rest of my layout has 24" radius curves, so I can run pretty much whatever I want, though my biggest locomotive at the moment is a Spectrum Light Mountain.