The Tunnel District is named because of 30 tunnels that are traversed in the next 27 minutes. They were originally constructed in 1904, and largely constructed by hand. AS the train curves back and forth, one gets great views of snow-capped mountains and also the Gross Reservoir. Water comes from South Boulder Creek, and from West of the Continental divide by means of a water tunnel! The Moffat Tunnel, 6.2 miles of scenic blackness," was built from 1923-late 1927, and the first trains passed through in 1928.
Semi-commercial note: At the Grand Junction stop, I went into Dave's Depot Store and bought "A Guidebook to Amtrak's California Zephyr: Denver to Salt Lake City," by Eva J. Hoffman. (Flashing Yellow Guidebooks.) I wished I had had it from the beginning, but it is helpful to me as I review my photos.