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′9 1/4″||3/4″|
|Class 7 track||4′8″||4′9 1/4″||1/2″|
|Class 8 track||4′8″||4′9 1/4″||1/2″|
|Class 9 track||4′8 1/4″||4′9 1/4″||1/2″|