What I would love to know is if it is possible for a 3 rail high railer to become a Master Model Railroader.
Yes. I know of at least 1.
By 3 rail high railer I mean a hobbyist with a layout that has 3 rail track and toy-like accessories much bigger than O Scale, like the Postwar Automatic Gateman and Animated Newstand, integrated artfully in realistic settings and scenery on the layout.
Possible
A lot of the qualifications for MMR have nothing to do with the actual layout, e.g. Volunteer or Official.
Only 1 of 3 pertaining directly to the layout would have to be achieved: Engineer Civil, Engineer Electrical, Scenery. You'd have the check the actual specifications and requirements for these to see what you could do.