Optical track is a recording unit of data on the disc. The information stored on CD-ROM is arranged according to certain rules, and is shaped like an "Optical track", called "light rail" in spiral shapes. The data from the directory inside the CD-ROM (TOC, Table of Contents) is recorded in the starting address of the number of consecutive logical sectors of an Optical track. The audio CD in a song is corresponding to a light rail, therefore, there are many light rails. CD-ROM light rails are at most 99 in number.
The minimum length of Optical tracks
The minimum length of each light rail is indicated by Time: 4 seconds, or 300 sectors.
This value is sometimes referred to as "threshold." Less than 4 seconds, light rail, if it can not be used as light rail, it might be a "bad Optical track"