Definitions for "Narrow Gauge"
A narrow Gauge railway has track with 3 foot 6 inch (1067mm) spacing between rails.
Railroads that were built with their rails spaced closer than the 4-feet 1/2-inch standard gauge. Two-foot and three-foot spacings between the rail heads were the most common in this country, particularly in the 1880-1900 period.
a railroad track (or its width) narrower than the standard 56.5 inches