VistaDomeScott posted:072 is minimum for 21" cars. A train that nice calls for some real estate expansion and wider curves.
I run Atlas Horizon/Comets through O63 curves (and at speed) with no issue, but that is the only full-scale rolling stock I have owned. The overhang would probably be distasteful to some, but I had to make some compromises for the space I had. The outer track is O72, and all the turnouts are Atlas #7, (except for the Atlas wye in the yard, along with some O81 curves). My next layout will have at least O81 ruling curves, perhaps even O90.
