The Southern New England Trunk Line Trail (aka the "SNETT") was designated as a National Recreation Trail in 1994. It is built upon a segment of the former right-of-way of the New Haven Railroad's Midland Division, which operated between Boston and Putnam, CT.
Today, it runs nearly uninterrupted for 22 miles between Grove Street near the southeastern edge of Franklin State Forest and the Connecticut state line in the Douglas State Forest. Although the trail is open and very scenic, note that much of it is currently unimproved with heavy overgrowth best suited to hiking and mountain biking.
The SNETT is the eastern leg of the larger Titanic Rail Trail (once known as the Grand Trunk Trail), which is planned to stretch more than 60 miles through south-central Massachusetts. Across the border in Connecticut, the trail joins the Airline State Park Trail that travels southwest for 27 miles to the town of Windham on the banks of the Willimantic River.
There is a break in the trail in Blackstone from Route 122 at Castle Hill Way to the west side of Factory Pond, and a shorter break in Millville at the Triad Bridges. There's also a tricky crossing at Worcester-Providence Turnpike/Route 146 and Route 146A. Use caution when attempting to link the broken trail segments and NEVER USE ACTIVE RAILROAD BRIDGES FOR CROSSINGS. Though most of the trail is unpaved, short sections have been paved and there are plans to pave more.
In Franklin access the trail by taking I-495 to Exit 16 and head toward Bellingham. After about 3 miles turn right onto Grove Street; the state forest entrance is about 0.25 mile on the left. In Douglas, take Route 16 W off Route 146 and go about 5 miles until you get to Route 96; turn left. The state forest entrance is about 0.25 mile on the right.
You can access the SNETT in other places where the trail crosses roads.