This line was originally built by the Denver, Northwestern and Pacific Railway, which was incorporated in 1902 to build a line west out of Denver to Salt Lake City, UT, on the opposite side of the Continental Divide. By this time, both Pueblo, CO (via the Denver, Rio Grande & Western) and Cheyenne, WY (via Union Pacific) had railroad lines heading west across the divide; it seemed logical that Denver should have one as well, and the DN&P was created for this purpose.
Construction on the line began in 1902, reaching the Continental Divide at Rollins Pass by 1903. By 1913, due to the difficulties of laying track in such mountainous terrain, the DN&P went into receivership and emerged as the Denver & Salt Lake Railroad, but it was for naught: construction on the line ultimately failed to reach Salt Lake City, only covering less than half the entire distance.
The climb over the top at Rollins Pass was fraught with numerous switchbacks, steep grades, and many instances of severe snow storms. These alone would seal the fate of the line over Rollins Pass, as the Moffat tunnel was built in 1928 to "replace" the tracks through the troublesome geography.
Today, the Moffat Tunnel is still in use by Union Pacific. The now-abandoned right-of-way over Rollins Pass branches from the UP line just east of the Moffat Tunnel's east portal, and then heads up to Rollins Pass via a series of switchbacks and tunnels. It then comes down the western side and meets back up with the active UP line at Winter Park. The entire abandoned route can be traced by following both Rollins Pass Road (on the eastern side) and Corona Pass Road (on the western side).