Old railroad grade turned into a well groomed, wide double trak. There are quite a few sections where it is quite rocky. This trail is closed in the winter, so call the Verlot Ranger Station for current trail conditions. Also on the warm sunny weekends, be prepared to meet lots of hikers and horseback riders. Now on to the good stuff. The trail used to be a rail line to the mining town of Monte Cristo. Thus, you never climb more than 8 or 9% grade. But it does climb, the whole way. Once up there, you can wander the old town site for awhile and take a breather. Coming back is a lot of fun, you can get going pretty fast in some sections and fly through the patches of rock. For more advanced riding, you can try and forge the rivers that cross your path instead of hitting the bridges - in fact one of the bridges was washed out some years ago so unless you want to hike across some narrow footbridges carrying your bike, you may just want to ride on through.
North from Seattle on HWY 9, turn right onto HWY 92 and go east to Granite Falls. At the end of HWY 92, turn left on the Mountain Loop HWY. Follow this until you hit the parking lot at the end of the Pavement. Be forwarned, this is a USFS parking lot and you do have to buy the Trail Pass before hand.
http://www.singletracks.com/item.php?c=1&i=1759