I've heard of people going that way to Concord before. But it wasn't obvious where the trail might have continued from there. There did seem to be trucks parked on top of railroad tracks a block or so down, and maybe if there weren't trucks you could follow the railbed further. (Looking at a map, though, a shorter but more trafficky way to go would probably be to just follow Mass. Ave. or equivalent from Lexington Center to Concord, if that's really what you were doing.)