The Snoqualmie Tunnel is at the top of the trail. It's 2.3 miles long, and even though you can see a tiny point of light in the distance, it's absolutely pitch black when you shut off your headlamps. We couldn't see our hands in front of our faces. Bring a powerful headlamp. You'll need it. Even with the headlamps, if we looked up from the ground in front of us and focused on the light in the distance, we'd quickly start wobbling and running off the trail toward the walls. It's crazy, but really fun.
After the tunnel, we made good time down the rest of the 22 mile trail. We hit some rain along the way, but we were prepared with rain jackets and it wasn't too bad. All in all, it was a pretty cool trip, although it wasn't nearly the workout I had expected. The road is extremely well maintained and it's very smooth and flat since it was a railroad grade. We finished the ride in well under three hours.
While we used two cars to avoid riding both ways, we found out there is a shuttle that runs back and forth so look into that if you want to follow in our tracks.
A few more pictures are available in the photo gallery under Outdoors > Iron Horse Trail.