I've only seen a small portion (8 miles) of what's to be had here, but the terrain is varied and rumored to contain MUCH more. Expect lots of rock and rooty New England singletrack with some decent riding thrown in. Very wet in the spring and after rains, but otherwise, well-ridden trails that are alot of fun. If someone goes deeper, please let us know what it's like!
Get off the Pike in Auburn and head west on Route 12. At the intersection of Route 20 and Route 12 in Oxford, park in the Wal-Mart parking lot. On the Route 20 side of Cumberland Farms across the road, enter into the riverbed and ride until you hit the road again (2 mi). Bear left to Route 12 and go a mile down Rt. 12 to Clara Barton Road on your right. Take a left towards the Mass Elec plant and take the steep climb on your right. This leads you to the Power Lines where you can ride to your heart's content. The real riding is taking a left into the first singletrack on your left.
http://www.singletracks.com/item.php?c=1&i=1722