Fort Mason in San Francisco. Built in late 1914, it was the western extension of the old State Belt Railroad, connecting the Port of SF to the Presidio. The Panama-Pacific Exposition of 1915 was about to go down, and they needed a way to get the thousands of tons of building materials from the port over to the marina. It also made it much easier to ship military people and materiel into the Presidio.
There's not much left of the Belt Line now. You can still see some tracks in the pavement near the east side of the Fort Mason hill. The tunnel is most of what's left, and makes a fun short jaunt into San Francisco history. Some photos from a recent expedition with a friend of mine.
The west exit, near the Marina Safeway: