In 1881, the line reached Durango and a short time later, a branch was built up the Animas river valley to the mining town of Silverton.

However the San Juan Extension experienced an unexpected "boom" in freight traffic due to growth in the oil industry around Farmington.

In 1970, a 64-mile segment between Antonito and Chama was purchased by the states of Colorado and New Mexico, and subsequently began operating excursion trains as the Cumbres and Toltec Scenic Railroad.

Meanwhile, the rest of the route from Chama to Durango, including the Farmington branch, was abandoned and the dual-gauge Alamosa–Antonito section was converted to a traditional 2-rail standard-gauge line, becoming the last 3-rail portion of the D&RGW system to be eliminated.

[5] The narrow-gauge portion between Antontio and Chama continues to operate as the Cumbres & Toltec Scenic Railroad with two trains (one in each direction) traversing the route each day during summer months.

[6] At the ghost town of Pagosa Junction, also known as Gato, a small portion of narrow-gauge track along with a steel truss bridge are still in place.

Almost abandoned in the 1960s, the branch was retained by the D&RGW to serve the tourist industry and the railroad operated a seasonal passenger service during summer months.

Much of the reason for this isolated change of gauge was that the Southern Pacific was contemplating extending to access coal fields in the northern San Juan basin, had surveyors working there, and had incorporated a subsidiary, the Arizona & Colorado Railroad Company, for this purpose.

After World War II, a “boom” in the oil industry around Farmington brought substantial freight traffic, which contributed to the lines viability until the early 1960’s.

[14][16] Several short-lived railroads were built by logging companies, with varying degrees of involvement with the D&RG, to harvest timber on both sides of the main line.

Typically, they would set up a sawmill at their junctions with D&RG, harvest stands of Ponderosa pine nearby and then extend spurs of six miles or so per year, as marketable timber in each area was exhausted.