Alpine Tunnel is a 1,772 ft (540 m) narrow gauge railroad tunnel located east of Pitkin, Colorado, on the former Denver, South Park and Pacific Railroad route from Denver to Gunnison. At an elevation of 11,523 feet (3,512 m), it was the first tunnel constructed across the Continental Divide in Colorado and was the highest railroad tunnel in North America at the time of its construction in 1882. The line was abandoned less than 30 years later, in 1910, after a series of accidents and problems in and around the tunnel. Now the tunnel is sealed, and the remaining trackbed serves as a trail for hikers and bicyclists.

The Denver, South Park, and Pacific Railroad was a historic 3 ft narrow gauge railroad that operated in Colorado in the western United States in the late 19th century. The railroad opened up the first rail routes to a large section of the central Colorado mining district in the decades of the mineral boom. The railroad took its name from the fact that its main line from Denver ascended the Platte Canyon and traversed South Park, hence its popular name "The South Park Line." Despite its lofty goals, the line never connected itself with the Pacific or any transcontinental line, apart from its terminal at Denver Union Station.