I actually went looking for some single track trail running. I found Lime Kiln on a site I found by googling "Single track trails". As far as single track, there are some single tracks that are pretty cool, but they are offshoots of an old forest service/fire trail. If grass growing on a gravel road is your thing, go for it! It would be great for a group run!
I was disappointed that there were slides that closed the trails. I could have climbed over them...my dog was halfway there...but I only had limited time.
I accessed the trail through Granite Falls, WA, which I think is the southernmost part. I wouldn't recommend driving 70 miles one way to go trail running, if single track is your thing, but if you happen to be in Snohomish or Lake Stevens for the day, why not take a short jog to Granite Falls, and try it out?