From Littleton to Woodsville, the Ammonoosuc Rail Trail is a 29 mile dirt biking route on an old railroad grade along the Ammonoosuc River. The route parallels US302/SR10.
The Littleton dirt biking trailhead is on Industrial Park Drive under US93.
The Woodsville dirt biking trailhead is on Highland Street at the US302 intersection.
The town of Lisbon is about the midpoint and makes a nice place to break (ATVs are allowed on the local roads to access the gas station).
It is also possible to follow the Blackmont Line, which branches off from the Ammonoosuc Rail Trail in Woodsville. It follows SR 10 3.9 miles south and ends in North Haverhill. The trailhead begins across from the Aldrich Store (food and gas) in N. Haverhill, ask for parking directions at the store.
Overall, Ammonoosuc Rail Trail is the 7th most popular dirt bike trail of all 18 dirt biking rides in New Hampshire.