Denver, Northwestern & Pacific Railway

This is the historic Moffat Road, built by David Moffat to connect Denver to the west, which eventually resulted in Denver becoming the large, important city that it is today. Building it was a heroic undertaking, requiring numerous short tunnels. The highest stretch, over Rollins Pass, was only intended to be temporary until a long tunnel could be dug under the Continental Divide. It required snowsheds in much of its highest elevations, and was fraught with hazards, accidents, and frequent closures in winter.

The traverse over Rollins Pass was replaced by the Moffat Tunnel in 1928, resulting in abandonment of that portion of the line. The lower-elevation remainder from Denver through the Moffat Tunnel remains in heavy use today for freight and passenger trains, and includes 31 tunnels of varying length. Tunnels 1-30 are original, and the 31st is the Moffat Tunnel. The Union Pacific Railroad calls it the "tunnel district". Portions of the high-elevation Rollins Pass route are open today as a 4WD road, though it is not continuous due to the closure of the Needle's Eye Tunnel.

You can travel the current portions of this route on the Amtrak California Zephyr passenger train, which passes through spectacular mountain scenery that can only be seen from a train, including 31 tunnels.

More information: https://en.wikipedia.org/wiki/Rollins_Pass