The California Zephyr enters the west end of the 6.2 mile long Moffat Tunnel that goes under the Continental Divide in the Colorado Rocky Mountains. The Moffat Tunnel's West Portal is at an altitude of about 9100 feet and adjacent to the Winter Park ski area.
Here is the east portal of the Moffat Tunnel.
This is shot out the back window on the train.
The window is very dirty on the outside, and the sunlight is lighting up all that crapola and washing out what can be seen in the bottom 1/3 of the shot, and the tunnel entrance is in shadow because the light direction sucks.
On the left we can just make out some white water in South Boulder Creek.
The smaller pioneer tunnel they dug next to the made tunnel was made a water tunnel that carries water from the western side of the Continental Divide. South Boulder Creek originates on the eastern side of the Continental Divide and joins that water from the western side just after it emerges on the Eastern side of the tunnel.
South Boulder Creek flows into Gross Reservoir part of Denver's water supply.