The height of the rail profile in thousandths of an inch: code 100 is 2.54 mm high, code 83 about 2.1 mm and code 75 about 1.9 mm. Lower codes look finer but need wheels with lower flanges.
Peco Streamline flexible track comes in code 100 (SL-100) and code 75 (SL-100F), both 914 mm long.
Peco Streamline 00/H0: articles, geometry and the track planner