In the extreme southeast of the San Juan Mountains is a highway pass called Cumbres Pass, elevation 10,022 feet. This pass is traversed by SH 17 and the Cumbres and Toltec Scenic Railroad as well as Cumbres Pass, which is the highest mountain pass reached by rail in the United States.

Cumbres Pass is on the Continental Divide Trail (CDT) and represents a transition between states for the CDT hikers. Northbound hikers head toward San Juan Mountain Wilderness in Colorado while southbound hikers enter the Carson National Forest in New Mexico.

Rocky Mountains Region

Southern Rockies Ecoregion

The railroad line was built in the early 1880s by the Denver and Rio Grande Railroad as part of their San Juan Extension line from Alamosa, Colorado to Durango, Colorado. The railroad has a steep, 4% grade approaching from the west, so additional helper locomotives were usually run and often still are on trains from Chama to Cumbres Pass.

The facilities at Cumbres pass were built by the railroad to support the turning of the helper locomotives for their return to Chama, as well as to provide water to locomotives after the climb. Some of the structures at the pass, including the station building, were demolished when the highway was realigned. Regular passenger service was provided by the San Juan Express, running from Alamosa to Durango, and was discontinued in 1951, although numerous rail-fan specials operated over the line until the Fall of 1967 and freight service on the line ended in 1968.

Since 1970 the railroad from Chama to Antonito has been owned and operated by the Cumbres and Toltec Scenic Railroad which operates trains over the pass during the tourist season.

The original section house, car inspectors house, and part of the old snowshed remain on Cumbres Pass and are maintained by the Friends of the Cumbres and Toltec Scenic Railroad. At present, no one lives at the summit of Cumbres Pass, though when the railroad operated as a common carrier line, railroad personnel were stationed at the pass full-time.