There are no existing, pre-manufactured track systems available for Sn42 modelers. Most Sn42 modelers hand-lay their track.
Micro Engineering manufactures rail in various code sizes, both weathered and unweathered. Although the Fast Tracks jigs do not support it, Micro Engineering also makes code 40 rail, if that is of interest to you.
The NASG developed a standard for track. You can read all about it on our Engineering Standards page. By having a well-known, publicly-available standard by which manufacturers can produce their products for S-scale, we ensure compatibility between brands, so that you can buy track from one manufacturer, and be assured that the locomotive that you bought from another manufacturer will run on that track with freight cars you bought from yet another manufacturer. If you are going to hand-lay your track, be sure to study those standards.
Rick Blanchard has a wonderful web page that lists track spacing, rail sizes, brake piping, and other dimensional data, including for S-scale. He also has a web page listing how rail sizes related to the various scales, and another web page on how prototype track curvature relates to scale proportions. There is also an Excel data file listing how real world dimensions translate to S (without having to pull out the calculator).
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