Code 83 vs Code 100 Track

Rail code is the height of the rail itself, measured in thousandths of an inch, and it has nothing to do with the distance between the rails, which is gauge, or the scale of the equipment running on it. Code 100 rail is 0.100 inches tall. Code 83 rail is 0.083 inches tall. Both are standard HO gauge at 16.5 mm between the rails; only the rail's height changes between the two codes.

Almost certainly yes if the equipment was made in roughly the last few decades with modern, scale-profile wheels. The real risk sits with older equipment, or with cheaper equipment carrying older-style deep flanges, where the flange can strike the taller spike detail or tie plates molded into Code 83 track before it strikes on Code 100. Check any specific locomotive's wheel profile against the manufacturer's specifications before assuming compatibility, particularly with vintage rolling stock inherited or bought secondhand.

Real HO mainline rail scales out closer to Code 83's height than to Code 100's, so a Code 83 rail profile reads as proportionally correct next to 1:87.1 equipment in a way that Code 100's taller rail does not. Modelers building a realistic-looking mainline generally prefer Code 83 for exactly this reason, while modelers running older rolling stock, building a first layout on a budget, or prioritizing reliability over rail-height realism often still choose Code 100.

Yes, with a transition. The two rail heights physically join with a graduated rail joiner built for that height difference rather than a standard joiner, since a standard joiner is not built to bridge a 0.017 inch step in rail height cleanly. Many modelers run Code 100 on hidden staging or a benchwork edge that takes rough handling, and Code 83 on the visible scenicked mainline, joining the two with the appropriate transition piece rather than forcing a plain joiner across the height change.

For a new HO layout running current-production equipment, Atlas HO Code 83 Super Flex Track 36 inch Section is the more common default and the more realistic-looking rail. Reach for Atlas Code 100 Nickel Silver Super-Flex Track 3 foot Section HO Scale if you are running older equipment with deep flanges you are not planning to re-wheel, building hidden staging where nobody will ever see the rail height, or simply matching an existing Code 100 layout you are extending. See the closely related question of rail height one step down at Code 75 vs Code 83 track.