Trying to find out what ON30 locos will be able to use a Atlas Turntable?
IIRC, the Atlas TT is 9", so any of the locos that have a wheelbase of less than that will fit. I'm sure that includes the Forneys and the Mogul, the Climax, the Shay, and probably the inside-frame 4-4-0. Most likely the 2-8-0 and the 2-6-6-2 won't fit. Others may chime in with more data.
Most of the smaller engines will fit the new Bachmann Turntable too according to their ad on page 5 MRR June.
TOF
I put one on the layout because I picked it up cheap and where the table wound up was right over the seam of two "domino" layout sections.
I think the 2-8-0 just fit, but I've long since sold both of mine. The 2-6-0 fit, the Forney, both 4-4-0s, and of course the Shay and Climax. 2-6-6-2 is of course too long. I had a 4-6-0 here by accident but I didn't take it out of the box before I sent it back, but I suspect they're too big.
However I did find if you want a turntable that is long enough and don't want to cut a hole in your table's top, there is a precident for a turntable that actually sits on top of the ground with the track ramped up to meet it. Several pictures in the Catskill Mountain Railway book show it. I'm figuring to make one using some Micro Engineering HO 30' girder sections. It only needs to be about 12" to fit the 2-6-6-2, if the couplers and pilot overhang a tad that's okay.
I suppose a guy could cheat and make such a table on the deck of the Atlas table, and hide the base of it... the ground would rotate with the deck but that might be an acceptable compromise.