It ran in people's backyards along 6A in Truro and Provincetown and through a lot of beachfront property, so after that much postwar buildout there's nothing left to preserve. Except for a missing bridge over a marsh, it's fully intact up to Commercial St. in Wellfleet about 2 blocks north of the town pier (look for the conspicuous "Railroad Ave." with no railroad near it). Rail trail ends 2-1/2 miles short of there.
If you look past Wellfleet, though, it's only sorta traceable on a map. Couple small pieces of public trail, then a lot of the rest is chewed up by private driveways and backyards of the uber-rich. It'll never be walkable without trespassing on a whole lot of very special private property. Not sure why those last 2.5 miles of trail are held up, as it's free and clear the whole way. Probably some persistent NIMBY abutters gumming up the works. Getting that last leg to Wellflett does get you right by the beach, the beachfront density, and a considerably bike-friendlier street grid than Route 6. Would increase the trail's utilization immensely to get it that far.
Google seems to think the end of the current trail is a happening-enough place.