Anton Anderson Memorial Tunnel in Anchorage, Alaska
Built in 1943 as a railroad tunnel to connect inland Bear Valley with the small port city of Whittier, the Anton Anderson Memorial Tunnel was built beneath Maynard Mountain and is now not only the longest highway tunnel in North America, but also one of the only ones to service both rail cars and automobiles, and is almost certainly the only one to use jet engines to keep the air fresh.
The remarkably tight, one-lane highway running through the tunnel originally only serviced rail traffic through its two and half mile length. The passage was constructed by the military during World War II to act as Alaska’s main supply route for goods arriving in the protected deep-water port in Whittier. When the military abandoned Whittier in the 1960’s, the tunnel was made a part of the Alaska highway system and given a massive overhaul. The traditional trestle tracks were replaced with a solid concrete path that brought the rails flush with the ground so that now cars could use the long tunnel as well, making it one of the only dual-use tunnels in the country.
To facilitate this new civilian purpose, a number of safe houses were built along the tunnel in the case of wrecks or natural disasters as well as pull off and turn around points that make travel through the expanse seem just a touch less claustrophobic. To maintain air quality in the underground tunnel, actual jet engines were placed at each end of the tunnel which force fresh air through a series of vents along the road.
Given that the road is only able to accommodate a single line of traffic, going only one way, at any given time the Anton Anderson Memorial Tunnel is now controlled by a sophisticated computer system that directs traffic at regular intervals with cars heading into Whittier leaving every half hour, and cars leaving Whittier allowed through on the hour each hour. Trains make it through when they can.