The Rio Grande pioneered narrow gauge in North America and once had the largest 3ft gauge system in the United States, peaking at 1,861 miles in 1889 (including 226 miles of dual gauge track). The Denver & Rio Grande Railway company was founded in Denver in 1870 by general William J. Palmer, his objective was to build a railroad from Denver southward to the border with Mexico at El Paso and westward to Salt Lake City in Utah. Its expansion to the south having rapidly been thwarted because of competition with the Atchison Topeka & Santa Fe, the Denver & Rio Grande turned to the west to serve the mining communities of the Colorado mountains, then booming (exploiting coal, silver and other ores). The Rio Grande system extended to the west and south of Colorado to Durango and Grand Junction, then in Utah to Salt Lake City and Ogden (originally by an separate company, the Denver & Rio Grande Western, later the Rio Grande Western), and in New Mexico to Espanola and Santa Fe.

    The decision to build narrow gauge had been taken for economical reasons. The rolling stock, smaller than for a standard gauge railroad, would be cheaper and the tracks would be easier and less costly to built, because the right-of-way could be narrower and the tracks could follow more closely the terrain with tighter curves, limiting the number of bridges and tunnels needed. But competition with other standard gauge railroads and the difficulties and cost of transfer for interchange traffic from narrow gauge cars to standard gauge cars and back, forced the Rio Grande to add a third rail to some of its narrow gauge lines (dual gauge tracks) as soon as 1881 and eventually to convert them to standard gauge. With the completion of the Denver - Ogden standard gauge mainline through Tennessee Pass and Glenwood Springs in 1890, the old narrow gauge transcontinental mainline through Marshall Pass and Gunnison was relegated to a secondary role. The Rio Grande narrow gauge system continued to decline as more lines were converted to standard gauge and narrow gauge lines were abandoned, soon to be limited to a few branch lines. However these timeless lines soldiered on until 1968, when the last D&RGW narrow gauge freight trains ran on the Alamosa - Antonito - Chama - Durango - Farmington line. Afterward, only the Durango - Silverton branch line remained active, mainly for tourism purpose, until 1981 when it was sold, bringing an end to 110 years of narrow gauge on the Denver & Rio Grande Western.