The size of your model train track will depend on the scale or gauge of model train you choose. You need to make a decision on the type of track, either the track with it's own built in road bed or the plain track which you layout your own railway bed.
Model and toy trains are classified according to scale and gauge. “Scale” describes the size of a miniature in proportion to its full-size prototype. “Gauge” refers to the distance between the rails of the track.
The largest model trains are collectively referred to as “large scale” trains. These big trains often operate outdoors on what are called garden railroads, though of course they can be run indoors, as well. These models are offered in a range of proportions, including 1:32, 1:22.5 (called “G scale”), and 1:20. But all of them operate on Gauge 1 track, which measures 45mm between the rails.
The next largest popular scale is O (1:48 proportion; pronounced “oh”). Track in O gauge measures 1¼" between the rails. This gauge is used for both toy (non-scale) and model trains. Lionel’s O scale trains have been produced for almost 100 years and, at their peak in the 1940s and ’50s, helped introduce millions of children to their lifelong hobby.
S-Gauge track is narrower than O, has two rails and supports American Flyer and other S-Gauge cars. S-Gauge is recommended for train hobbyist and collectors who prefer a scale smaller than the traditional O-Gauge or O Scale line.
Slightly smaller than O scale is S scale (1:64 proportion). These locomotives and cars run on rails spaced 7/8" apart. Unlike their toy predecessors, today’s S scale models are as highly detailed as trains in other scales.
Overshadowing the larger scales in popularity are models built to be approximately half the size of O scale models. These trains are 1/87 the size of their real-world prototypes, and HO gauge track measures 16.5mm between the rails. HO trains are small enough to allow a satisfying layout in a compact space, say a 4 x 8-foot sheet of plywood, while still being large enough to show great detail. No wonder HO railroading is the most popular of the scales, with more than two-thirds of modelers making it their choice.
Smaller still is N scale. Rolling stock and locomotives of this size are 1/160 the size of their real-life counterparts. The track gauge is 9mm between the rails. N scale works well for modelers who don’t have a lot of space or who prefer to run trains through truly expansive scenery.
Even smaller are Z scale trains. Their proportion to the real thing is 1:220, and they run on track whose rails are just 6.5mm apart. How tiny are these trains? Well, a model of a real-life 50-foot locomotive measures just 23⁄4" in Z scale