North of Boston and Medford, the 2,600 acre Middlesex Fells Reservation (aka The Fells) has 25 miles of trails open to mountain biking. One of the more popular routes is the 6.3 mile NEMBA Loop (follow red signs from the main parking areas for the Loop).
To reach the mountain biking loop trailhead at Sheepfold Parking Area from US93, use Exit #34 or Exit #35 to Main Street (SR28). Head southwest on SR28 back under US93. The signed trailhead parking access road is on the right.
Overall, Middlesex Fells Reservation is the 31st most popular mountain bike trail of all 35 mountain biking rides in Massachusetts.