What is the smallest radius track that a 2-6-2 or 2-8-2 can run on - not a passenger train
That´s close to impossible to tell without further information. The ability to negotiate tight radii depends basically on the length of the rigid wheelbase, which in turn depends on the diameter of the drivers. 2-6-2´s and 2-8-2´s came in all sizes and gauges, just compare a D&RGW K-27 with a USRRA Mikado.
Can't promise anything, but IF I can get my PC at home to work online I can see what I can find. I recently bought a couple of books on NYC steam locomotives, some of the engine diagrams include the prototype's minimum curve. IIRC they give the min. radius as a degree rather than in feet - like "18 degree minimum curve".
I couldn't find anything on 2-6-2s, but according to the NYC steam engine book an H-5 2-8-2 had a minimum curve radius of 18 degrees 30', which in HO works out to something like a 45-46" radius curve.
Depends on which brand and which model, but I would say 18"R for stuff like Bachmanns smaller 2-6-2 engines, but the bigger 4-8-4 needs a 22"R and would be safer at 24"R. Most really big engine models need wider radius than that to work right.
Check the manufacturers product description, they will almost always include a minimun radius requirement.
Oops! nevermind, thought I was still on the MR site
Have fun with your trains