I have a Big Boy, Class A, Allegheny, Cab Forward, Challenger, etc. and my double track is 096 / 088. Spacing between the center rails is minimum 4-1/4" on the curves. One end of a curve enters a long trestle and I needed 6" straight prior to entering the bridge. Trust me, you need a lot of room on curves for those locomotives. O72 would be greater distance.
bruce