Yes, build as big as you can

The advice to build as big as you can is always good.
Do you people at SV have the builder's specifications for the Alco 2-8-2s? These would probably indicate maximum curvature that the engines were designed for.
For comparison the military 2-8-2s of WWII were suppose to take a curve of 25 degrees (slow speed!). This works out to 231.01 foot radius,
Many diesels can squeek through a curve of 21 degrees (274.37 foot radius) or better. The UP 844 and 3985 are designed for 20 degrees maximum curvature (287.94 foot radius).
Roadrail curvature is based up on central angle of a 100-foot cord. Brush off your high school trig and your calcuator with the trig functions and you can figure out the radius for any given curve.
Brian Norden