Perhaps the most common factor relevant to model trains is the gauge, but the scale standard addresses gauge and a host of other scale-specific dimensions. The National Model Railroad Association lists 47 scale standards for electric model trains.

In full-scale trains as well as model trains, the gauge is the distance measured from the outside of one track to the outside of the other track, in a straight line. The largest gauge is G (for “garden”), which measures 45 millimeters between the rails, and the smallest is the Z, at 6.5mm. The scale is the size of the train, as a proportion of the train it is modeled on. G trains are available in two scales, 1:32—meaning that the model is 1/32 as large as the “real” train, and 1:22.5. Z-gauge trains are 1:220 scale, which means that an actual 50-foot-long locomotive is 2 3/4 inches long.

In between the G and Z scales are the O, S, HO (for “half O”), and N. Typically, a model train’s scale standard also covers catenary (power-delivery line), loading gauge (height and width), grade (the angle of a slope the track may be on), curve radius, plus much more. There are many more NMRA scale standards, including “deep flange scales”, “proto scales”, and “finescale”.

More Info: en.wikipedia.org