What is the width of seat tracks on airliners?
The attachment rail/track width (gauge) is not fixed. It differs from plane to plane, and the chosen seat manufacturer obliges.

21" or 55 cm seems the average though.



Source: stanford.edu

Above image shows B717 cross section, you can notice the 3-abreast and 2-abreast both are on two rails. No high-res available.

Different plane models:



Some are wall mounted.