Golden Rule

[Sumpter250]

Well, surely rules are meant to be broken...

The Golden Rule is meant as a guideline for maximum dimensions for equipment for tunnel and bridge clearance.

I misinterpreted that as meaning carbody width and height. It is actually clearance width and height. 3.25 X 56.5" (standard gauge)=15.3', which is the width(15') of the NMRA Standards gauge in HO scale.
A carbody width and height ratio would be different, and would vary with gauge, car style, and car age.
There is a definite case for "That loco, or car, looks too wide for the track gauge" feeling. There is also a definite case for the relationship between car width-track gauge-physical stability. Running standard gauge cars on 3' gauge track/trucks, was done.......but there were "complications", and necessary care in loading, and slow speeds, especially on curves.
In the end, nothing will stop the Sag Harbor Shipbuilding and Drydock Company from building locomotives and rolling stock for their "31" gauge" New England railroad. And that, is the beauty of this hobby.....if the road is freelance, there can be some "fudging" (aesthetics) where dimensions are concerned. To each, their own.