John,
I like to add to what you stated with some real examples. I have run K37's or smaller loco's on a display layout owned by Sandy Webster in Seattle that has 12 inch radius curves. And yes the trains were very short usually no more than six cars. If you have a PFM Sumpter Valley Mallet its min. Radius is stated as 18 inchs, it will actual run on 17 & 1/2 inch radius with a short train, six cars (my display layout). It will pull a 12 car train just fine around 20 inch radius as on the Boeing Employees Model Railroad Club layout.