Sn42 equates to Sn3.5, as an "either or" expression of types. Sn42 meets the Sn3.5 character of modeling as both utilize HO locomotives/rolling stock and On30 locomotives/rolling stock modifiable to Sn42's 1/64th scale. Mechanical power units and wheel sets are generally HO or HO equivalent in size in order to fit within the desired locomotive shell and compliment gauge. Kit bashing abounds making this scale a very cost effective type to model. Track, turnouts and crossovers are HO specific and very reasonably priced. On30 track appears to be more visually correct than HO. S scale structures are required. Creativity is a key word in describing Sn42 scale. Purist lean toward the narrow gauge Sn3, Sn2.5, Sn2, and proto version modeling as they are more singularly gauge specific, some requiring hand laid track etc.