When I first moved to Boston, the Red Line went from Harvard to Ashmont (red because it’s close to crimson). Then the northern portion was extended from Harvard to Alewife. It was supposed to go to Arlington and Lexington, but those towns rejected the extension of rapid transit, which might bring riffraff into the leafy suburbs. Couldn’t have that!
So that took care of the northern end, but what about the southern? If you look carefully at MBTA maps, you’ll also see an extension beyond Ashmont. If you look more carefully, you’ll see that the path between Ashmont and Mattapan is actually a separate Red Line, known peculiarly as the Mattapan-Ashmont High Speed Line. It uses old PCC trolleys, not heavy-rail subway trains. And it’s the only transit line in the country to pass through a cemetery! (Keep your eyes peeled.)
Now you can watch a video of the High Speed Line in full 360°! Go do so; it’s worth the trip. Be sure to listen to the conductor’s announcements, and eavesdrop on the background conversation, all about the relevant history and lore.