From Littleton to Woodsville, the Ammonoosuc Rail Trail is a 29 mile route on an old railroad grade along the Ammonoosuc River. The route parallels US302/SR10.
The Littleton trailhead is on Industrial Park Drive under US93.
The Woodsville 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.
Note: The State Of New Hampshire requires a NH state registration tag on every ATV. An out-of-state registration tag is not valid in NH. They will issue a fine if you try riding without a NH tag.