Term: narrow gauge (railroads)
railroad track where the rails are spaced less than 4 ft 8½ in (1435 mm) apart. There are many common gauges narrower than standard, amongst them ... 3 ft (914 mm), which was the most common narrow gauge in the U.S.; ... Narrow-gauge lines are often found
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[Source: "Rail terminology" at Wikipedia (http://en.wikipedia.org/wiki/Rail_terminology)]