49 CFR 213.323: Track gage.

(a) Gage is measured between the heads of the rails at right-angles to the rails in a plane five-eighths of an inch below the top of the rail head.

(b) Gage shall be within the limits prescribed in the following table:

Class of track The gage must be at least— But not more than— The change of gage within 31 feet must not be greater than—

Class 6 track 4′8″ 4′91⁄4″ 3⁄4″

Class 7 track 4′8″ 4′91⁄4″ 1⁄2″

Class 8 track 4′8″ 4′91⁄4″ 1⁄2″

Class 9 track 4′81⁄4″ 4′91⁄4″ 1⁄2″