[3] It occupies an abandoned railroad corridor running for approximately 22 miles (35 km) with an eastern terminus at Union Street in downtown Franklin, Massachusetts and a western terminus at the Connecticut state line in the Douglas State Forest, Douglas, Massachusetts.

Despite its name, the trail does not follow any part of the Southern New England Railway, a railroad intended to run from Palmer, Massachusetts to Providence, whose right-of-way was partially constructed but never completed.

East of Blackstone, improvement work is proceeding west from Grove Street in Franklin.

At Prospect Street in Franklin, where a former overpass was replaced by a fill, a new tunnel was completed under the road in 2020.

Media related to Southern New England Trunkline Trail at Wikimedia Commons