49 CFR 213.53 - 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 -|
|Class 1 track||4′8″||4′10″.|
|Class 2 and 3 track||4′8″||4′9
|Class 4 and 5 track||4′8″||4′9