I don't recall ever seeing any published, accepted clearance standards for "0" gauge construction. For example: the minimum clearance height above the rails for all equipment, or the clearance to the side of the outside rail to clear obstructions or rolling stock on adjacent tracks.. (Realizing, of course, that this last "standard" would be dictated differently for straight track and for curves of varying diameter (radius in scale "0"). I also can accept that there would be different standards for 0-27 sized or semi-scale engines than there would be for full scale models. Another measurement that could be included would be the minimum (and maximum) catenary wire height for electrics.
Of course, a Big Boy rounding an 0-72 curve through a tunnel portal would require more clearance than most average engines, but I wonder if anyone has actually published some sort of standards? How about degree of incline for good operation between levels? For that matter, the degree of upward or downward curve in rails going from level to an incline?
Railroads used to have a "clearance car" with lots of adjustable extensions that could measure these types of restrictions on various track configurations. Perhaps it would be a nice idea to build such a car for model layout builders.
Paul Fischer