Get off 128 at Great Plain Avenue and head towards Dedham and you'll see what looks like a driveway just before the Dedham line.
It's actually the remains of an old road, one that now dead-ends at the soon-to-be-expanded 128. Park by the gate, and you'll see the old town-line signs. Look down toward the river and you'll see an official boundary stone (like this one). Then look at the phone poles and lines that still run along the side of the road - and continue across 128.
Could this be the old alignment of Great Plain Avenue before 128 was built?
And, at least for now, you can walk right to the side of 128 - where you can wave at passing motorists, some of whom might just wave back.