is there a min. radius track for Northern 4-8-4 With Operating Headlight & Vanderbilt Oil Tender - G.N.
They will run on 11" radius but it limits their ability to pull a train without derailing. I think Bachmann recomends 19" radius but 14-15" is probably the minimum for smooth opperation.
It's sold with 11¼r curves in the Empire set, and to be honest, I've never considered running it on anything smaller.
I would guess 11¼r is a safe minimum, any less may be iffy.
As they come they are already tight on gauge and need to be checked.
If you would have to tighten the gauge up to get it around a tighter curve then you probably will forfeit reliability over turnouts. I may also make the lokie hunt for the center of the track (waddle).
Still, it may be doable with some trial and error if it had to be done, but in that case, I'd look for a smaller loco.