(Edit: I started this before Jim P. posted...had to do a little bit of digging and Google Maps-ing)
Looks like the 9th Avenue Elevated in The Bronx. Rapid transit.
Long abandoned and most traces difficult to see unless you know where to look. The tunnel is still there (it went under the apartment buildings below street level), but sealed off. The street where the elevated portion connected to the Jerome Avenue line is blocked by the new Yankee Stadium, the only remnants obscured by trees on Google Earth. (if you look there, it's the blue-painted wall facing the Stadium as Jerome Avenue curves away from it heading south, and if you scroll back to the #4 subway on River Ave, you'll see the stub of the former connector sticking out from under the structure)
Pictures of the Ninth Ave El remnants
---PCJ
