Here's a clip from Wikipedia.org on the future of the Ashmont / Mattapan Line;
"On several occasions, the MBTA has proposed to replace the PCC streetcars either with newer trolleys or with buses, and has met with substantial community opposition on each occasion. The FY2017-FY2021 Capital Investment Plan, approved by the MassDOT board in June 2016, allocated $9 million to the line including $3.7 million for maintaining the PCC cars. The plan also allocates $5 million for "PCC Car Replacement-Alternative Service" which will be used for future funding should an alternate form of transportation be decided upon.[16] In 2017, MBTA began a $7.9 million project to overhaul the trollies and update the propulsion systems, but further problems such as fluctuations in power damaged the four operating trains in early 2018.[17][18] On February 5, 2018 it was reported that eight more trollies were going to re-enter service in 2019, the fixes will prolong the lifespan of the trains by another 7 to 8 years.[19] The MBTA also announced that it is studying alternatives again to running trolleys, draft recommendations are planned to be completed by the Spring."