The PCT actually goes over the Iron Horse trail tunnel. Also known as the Snoqualmie tunnel. Once the PCT reaches I-90 you can head east on a frontage road to the tunnel. Its less than 3 miles to the east opening, with about a 450' drop. The west tunnel opening is closer to the PCT but much harder to access. I do not recall any trail or path connecting the PCT to the west opening near rockdale creek.
Here is a map of the area in question. I also added the path from the PCT to the east tunnel opening.
http://www.hillmap.com/m/ag1zfmhpbGx...BiAgICguO6sCQw
I am surprised that the tunnel is not a more popular alternate route. FYI, the tunnel is over two miles long, You will want a flashlight. The tunnel path is smooth but has drainage ditches along the walls and water does drip down all along the tunnel.