This is one of the Mattapan-Ashmont trolleys. The trolley runs from Ashmont, at the South end of the Red Line, to the Mattapan station with about six stops in between (including Milton, which is my stop!). The cars are originally from the 1940's and were rehabbed for this trolley line. There's a lot more info here, but I wanted to share a quick little snippet:
This short trolley operation, unique in the MBTA because it still uses PCC trolley cars, runs from Ashmont to Mattapan, the end of the line. This route, only a few miles long, has 6 stops along its way to Mattapan. It was built on a railroad right of way to help speed trolley cars into Boston from the Dorchester area. At only 2.2 miles the line is one of the shortest existing trolley lines, but it is surprisingly scenic considering its location on the border of a major city.