Citation: L-Gauge - Track Geometry (Article), May 25, 2020.

A convention of maintaining 16-stud spacing between adjacent parallel track centres is very widely adopted and is strongly recommended. The 16 stud interval is also very common among many LEGO® baseplate sizes and therefore track elements can often be mounted to baseplates with connector ends flush to the baseplate edges.

Official LEGO train track has a tight radius of 40-studs, while 3rd party track comes in larger radiuses. They are generally a multiple of 16-studs larger than the official 40-stud radius.