@bob2 posted:O-72 is 36" radius. That is really tight for 2-rail scale. A typical scale Pacific won't make it without cutting the cylinders and frame. Also the tender drawbar must be extended beyond a realistic length. Most of us regard 48" (O-96) as a rock-bottom minimum for all but switchers and ore cars.
I get that with all the drivers flanged it would be a tight fit on O72.