Many Gauges & Even More Styles of Track
For model railroaders, there are more than a dozen gauges of two and three rail (sometimes four) track. The smallest is Z which is tabletop size and the largest in current production is G for Garden Scale. The width between Z Gauge rails is .25 inch. For G Gauge, it's 1.75 inches.
For every gauge of track there are numerous styles to choose from. Many are dated and out of production like the tinplate American Flyer Tru-Scale HO scale track pictured in this post. It was only produced for a short period of time during the late 1930s and early 1940s.
The thing is, if you can find enough of any style track to build a layout, it's still usable. Some styles are even compatible with others, although the look might not be pleasing to some of the more particular hobbyists. In any event, you can save quite a bit of hobby-related funds by buying clean used track versus new.