This is the replacement for the original Cascade Tunnel in Stevens Pass. Opened in 1929 back in the days of Great Northern, the bore is single track and nearly eight miles long. In order to address ventilation issues in a tunnel of such a length, this portal has a large shutter that closes while the train is inside, and fans provide ventilation to clear the exhaust smoke. I'm glossing over a lot here, but it is a very cool piece of engineering and history, so look it up!

Found this in some old photos about six or seven years ago and realized I hadn't posted it.