The portion of the Williamsburg line from Northampton through Florence and Leeds to just north of the Williamsburg line is fully accessible as part of the Northampton area rail trail network.
It's pretty much on the ROW apart from right at rt 5, and up at the Look Park rotary where it detours into the park itself before emerging again along the south side of rt 9 to follow the ROW above surrounding streets.
Some past disputes mean it ends at a pile of dirt just across the Williamsburg line, where a now-paved "goat path" runs down to the street. If one crosses route 9 and heads out of Haydenville on High Street it's clearly visible as a channel of smaller regrowth through the woods on the hillside, and more leading up to the lumber yard can be seen from route 9, there are increasing hopes of opening that, too.
Back at rt 5 in Northampton there's a box car being used as storage by an auto repair place, but it's not sitting on any remnant track. The track configuration in that area changed many many times over the years. The current crossing of route 5 is obviously not a rail remnant but result of a long ago battle with the auto shop. East of rt5 a sidewalk dogleg brings this to trail paralleling the active tracks and behind the Walgreens a newly constructed tunnel
under them to connect to the Mass Central's through truss bridge over the Connecticut River and about 10 total miles of converted rail ROW running through Hadley and Amherst to Belchertown.