I am confused. What was the question? Geometry is geometry, and the limits have mostly to do with truck swing and coupling distance. A 3- rail locomotive can turn sharper corners partly because when a wheel strikes a cylinder block it does not blow the circuit breaker.
Consider - if you take an HO model and enlarge it to 187% (or whatever that scaling factor is) you can also enlarge the radius by that same percentage.
What most 2- railers want is lead trucks and cylinders of proper size, tail beams under fireboxes, and cars coupled realistically close. That works out to about 70" radius or more.
If you buy, for instance, a "scale" MTH. Hudson, you will see no tailbeam at all, seriously undersize cylinders and lead truck wheels, and off- center piston rods. That is so it can get around 36" radius for 2-rail. If you run 3-rail, it can probably go O-54, which is slightly less than 36" radius, because the pilot wheels and tender can touch things without shorting.
That's all opinion, and may not track what the original question was.
