This is one of those things that's just fun to experience. It's also a great way to get over (or worsen) a fear of the dark. The 2.6 mi Snoqualmie Tunnel is part of the Iron Horse State Park's 110-mile long trail system that converted a bankrupted railroad right-of-way into a state park in 1980. The tunnel cuts directly under Snoqualmie Pass and offers a great experience for anyone looking for something unique to do (May-October).
It took us around 45 minutes each way. There were quite a few hikers and bikers making the trip going westbound, but we were pretty much alone returning eastbound because we left later in the day.
I climbed on top of the tunnel entrance on the west side to get some pics. It's a little tricky, but there are rebar steps on the north side that can be used to scramble up. The views to Granite Mountain are pretty good.
Bring a headlamp, some binoculars and a coat. Wearing something reflective for bikers approaching from behind would be helpful. The tunnel closes Nov 1st.