Aspen Tunnel was one of the greatest improvements made by the Union Pacific Railroad, saving 10 miles in distance from Leroy to Evanston. Work on the tunnel began on November 13, 1899, and was completed on October 9, 1902. It remained a single-track tunnel until the Altamont Tunnel was constructed in 1947 and completed on November 19, 1949. A side adventure to the tunnel from Mirror Lake Scenic Byway may be taken on an unimproved dirt road east of the Bear Town interpretive site.