Here is the East Portal of the Moffat Tunnel.
http://en.wikipedia.org/wiki/Moffat_Tunnel
It is the highest operating rail tunnel in the US, the 2nd longest, and the 3rd longest in the continent. (Stevens Tunnel in WA, and MacDonald in BC are longer). It's a former Rio Grande Western mainline, and is now owned by the Union Pacific, and is Denver's main rail gateway to the West. Further up the mountain one can periodically hear the massive exhaust fans that turn on after a train passes to clear the tunnel of the diesel exhaust (about one train can run every half hour for this reason. The noise of the fans is amazing, like an earthquake as it echoes in the valley. =)