For everyone's information, I just set up an HO 15" curve and ran my Bachmann 3 truck Shay through it. Even at slow speed she bucked off the rail coming out of it. Just sitting there with no power the tender and loco touch, but there is a little play, so I thought I got lucky. Running, whole 'nother story. I'll try 16". BTW the axles never popped out. (UPDATE 7/4: I ran the 3 truck Shay through the 15" curve again with my glasses on, certain all wheels were aligned first. It would not derail even at top speed. Can't say what happened but I'm adding Shay to the 15 club!)
Then I had success, no derails on the 15" HO curve with the following:
Bachmann 10 wheeler Baldwin.
Proto 2000 SW8
Proto 2000 FA1-FB1, hitched.
Hope this helps! -Rob
(from another forum discussion)