I feel the previous two comments are downplaying the current service quality on the Mattapan Line. As of the Winter 2024 schedule, the line runs
6-min frequencies on weekdays during rush hours, or roughly 9.5 tph. Middays still see 7-8 min frequencies. That's slightly better than each individual Green Line branch
(7 tph in 2022 during peak).
Sure, the Mattapan line doesn't through-run into downtown Boston, and the lack of timed transfer at Ashmont hurts
(though that's primarily a problem with the Red Line right now, and should have been seamless if RL worked properly). But given that Milton is much further away from downtown than the B/C/E branches, I'd guess Milton still gets a
faster trip per mile than Boston College, Cleveland Circle and Heath St. When comparing to services that do not through-run into downtown, the Mattapan Line's service levels should blow away even the vast majority of Key Bus Routes
(if not all), which do not have 6-min peak frequencies, and which run on streets with no grade separation, no signal priority and sometimes even no bus lanes.
I don't know what exactly makes the Mattapan Line not "very robust light rail service" or having "shitty frequencies".
This is before considering that it serves areas that are much less dense than either the GL branches or Key Bus Routes, so its riders are already disproportionately advantaged.