D500 posted:I'd say those were more 072 than 031.
O36 seems more appropriate. 
As to the question of what to run on O31 curves, I generally just use it for handcars, small motorized units (like snowplows, speeders, gang cars, burro cranes, etc)
While I've certainly got some PW style engines that may navigate O31, I also run larger stuff, so I wouldn't create a special loop just to run the smaller stuff, I'd just run on one of the larger loops.
Actually after thinking about it a little more, I did one time have an O31 dogbone that I ran elevated at the back of a layout in my old apartment years ago. I ran some MTH RailKing Subway sets on that, IIRC.
-Dave
