Leading a stack train, BNSF 5702 emerges into the sunlight after traversing the 7.8 mile long Cascade Tunnel on BNSF's Scenic Subdivision. A fan air intake can be seen on the far left of the photo. To ventilate exhaust gasses in the longest tunnel in the United States, two large electric fans provide positive pressure inside the tunnel. Clean, fresh air is ported through the south side of the tunnel wall, about 100 feet in. A large red and white steel door slides into place when a train is inside the tunnel. When the train gets within 1/2 mile of the door, it opens. After the train gets through, the door closes again and the fans ramp up to maximum capacity to flush the tunnel. Depending on the train, flushes can take up to 30 minutes.
The west end of the Cascade Tunnel can be seen here.