The Continental Divide sounds like an intimidating barrier to transcontinental travel, however on some rail lines such as the Southern Pacific's Sunset route the crossing is so sedate that it's not even marked. The Great Northern offers a bit more drama, but was still able to get over the top out in the open at an altitude not much higher than Denver. Colorado however is where the Continental Divide in the form of the Rocky Mountains lives up to the stereotype, creating a massive barrier to east-west travel. While most of the Brand Name railroads built their routes to the north or the south, the Denver, Rio Grande and Western decided to go through the Rockies, not around them and this ultimately took the form of two distinct route. The first ran northwest from Pueblo to the headwaters of the Colorado river over the Tennessee Pass. Unfortunately this meant a 180 mile detour for trains trying to get west from Denver so in 1922 the State government funded a project to tunnel through the Rockies on a more direct route. On my summer 2020 trip out to Denver I visited both of these crossings and you can find the complete set of photos here ( mirror ).
The Moffat Tunnel was built between 1923 and 1928 and runs 6.2 miles underground between a point west of Tolland, CO and the resort community of Winter Park. Due to the length of the tunnel forced ventilation is a requirement, even with comparatively less polluting diesel locomotives. As the comparatively populated Winter Park would object to a constant haze of locomotive exhaust, the fan plant was located on the east side of the mountain seven miles down a dirt road from the nearest state highway.

Unlike the East, large portions of Colorado is public land and therefore
the East Portal is a popular trailhead with ample parking, day or
night.
The Moffat Tunnel route had actually been first cut by the
Denver and Salt Lake Railway,
that used an extreme switchback route over the Rollins Pass that
ultimately proved unsustainable. This still eased the way for the
Moffat Tunnel project that just had to bypass the bit over the Rolls
Pass. Today some of the cabins used by the tunnel work crews are still
standing and were recently made a state historic landmark.
The original tunnel headhouse consisted of two parts, a pressure door
and two large squirrel cage blowers with the gatehouse structure
supporting the door and the two boxy towers behind it the blowers. The
door would prevent blower air from just rushing in/out the eastern end.
In 1988 the Rio Grande purchased the Southern Pacific with the resulting
combination being purchased by Union Pacific in 1996, who operates the
tunnel to this day. With the UP's original main line through Wyoming to
Ogden, UT offering lower grades and more capacity, the former DRGW
route now sees a much reduced amount of traffic with most road freights
terminating in Grand Junction, CO leaving Amtrak as the primary user of
the line in its original purpose as a shortcut to Salt Lake City and
beyond.
The Moffat Tunnel itself was built by the public improvement district
and leased to the railroad. Bronze plaques at the east portal attest to
the principles involved.
The original fan plant was upgraded to modern standards some time in the
1980's with an entirely new plant built directly behind the first. A
new door system was installed and the squirrel cage blowers were
replaced by axial turbine units. To avoid polluting Winter Park the
fans suck air towards the east end with large exhaust horns attached to
the comparatively small electrically driven fans. There are two primary
fans located on either side of the tunnel extension.
As giant fans are
long lead items, a spare fan motor is on site in case of a failure.
A third fan is available to blow air from the east in the face of slow
moving eastbound trains that might become bound in their own exhaust
plume. Line speed is 40mph compared with 25mph in the similarly
operated Cascade tunnel.
Direct drive diesel backups are provided for each fan.
A 10 foot wide pilot tunnel used in construction of the main bore was
repurposed into a pipe to transport water from the Colorado River
headwaters on the western side of the Continental Divide to the City of
Denver on the eastern side. The water can be seen her exiting the
mountain next to the rail line.
The fan plant requires a substantial amount of electric power which is supplied to this substation.
I timed my visit to coincide with the afternoon passage of Amtrak Train 6, the eastbound California Zephyr from San Francisco. Power on the head end consisted of P42'sDC
#17 and #167. At 9239 feet in elevation the Moffat tunnel is the
highest point on the Amtrak network as well as one of the highest main
line passenger routes in the world.