Either use LRVs, Bus Rapid Transit or extend the subway line.
The Red Line has already been extended, but in a different direction. The Ashmont-Mattapan shuttle is now a branch off of the Red Line.
PCCs operate very reliably in San Francisco, Kenosha, Philadelphia, providing regular transit service, on lanes mixed with automobiles, a more demanding environment that the private-right-of-way Mattapan - Ashmont line. Well maintained, they can be good for another 50 years.
Don't forget, the Isle of Man's Douglas-Laxey-Ramsey interuban is providing regular service with 120-year-old original equpment rolling stock. And the environment is no less demanding than Mattapan - Ashmont.
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