I have a pair of them and they're pretty long. The car is 13" long and the trucks are at the extreme ends of the car (correctly). I never tested mine around anything smaller than 36" radius (O-72) as they're 2-rail cars, but there's going to be quite a bit of underhang (inside of the curve) going around 18" radius (O-36) with truck-mounted couplers. That said, the Atlas site says they'll make it around O-31.
Hope this helps.