"As for taking 18" radius curves, it's true that this is a requirement for many On30 modelers, but I doubt if the prototype could handle similar curves if scaled up, and the prototype 4-4-0's were never asked to."
I agree totally, just as Bachmann's HO scale EM-1 2-8-8-4 was never asked to negotiate the prototype equivalent of a 24" radius curve.
Bachmann is smart enough to understand that the market is extremely small for an EM-1 that has a 36" minimum radius curve restriction.
There are compromises when making an EM-1 2-8-8-4 take a sharp curve, and there is no doubt that those compromises detract from the proto fidelity of the model.
But the thing is this: that's where the sales are--the vast majority of On30 modelers are using 18" (or maybe 22") radius curves, so that has to be taken into effect when producing a model for the market. Scale purity/prototype fidelity is a worthy goal for a modeler, but importers and manufacturers usually have to compromise standards to produce a product that can actually cover costs and make a profit. The Fn3 4-4-0 will go around a 2' radius curve (I know-I own one). It looks a bit silly doing it, but it will reliably track around those small curves.
If I had the room (and the money), I would be very happy modeling in On3. I would love to a huge layout, with large radius curves and a roundhouse full of beautiful brass models. It would also be fun to have an operating session on that dream layout with Jennifer Love Hewitt.
Jeff C