There's a case to be made that neither the Riverside or Needham sub-branches west of Newton Highlands need full frequencies (especially if Indigo to Riverside happens).
It's
so annoying when crayoning because... yes, Needham probably does not need 6-min headways, and Riverside maybe doesn't either if there is Indigo service. But... they probably merit something
more than half service. If the core takt is kept at 6 minutes, then
maybe half-frequencies of 12 minutes could be workable. But if that takt increases to 7 minutes, then you're at 14 minutes, which is no good.
Sending Needham vs Riverside to Kenmore vs Huntington has arguments in all directions. Maybe send Riverside to Huntington and let travelers who need to get to Kenmore take Indigo instead. Maybe send Needham to Huntington to maintain access to similar areas as the current commuter rail, as
@HenryAlan suggests. And then there's the question of sending Huntington trains to Park vs South Station, and doing so while maintaining legibility and show-up-and-go frequencies for each destination pair. (i.e. Don't build a 6 tph frequency cake to Needham with 2 tph to Kenmore-Park, Huntington-Park, and Huntington-Seaport with 30 minutes between each train.) Like I said --
so annoying.
My take is to run each branch at three-quarter frequencies, into a single trunk line (I prefer Huntington, built as a modern LRT subway, able to take maximal advantage of supertrains and the mostly-sealed ROWs to Riverside and Needham, and boosting headways along Huntington); if there are concerns raised about service reductions between Riverside and Kenmore, then layer on a half-frequency Riverside-Kenmore short-turn that doesn't through-run.
That would give 4-5 minute headways east of Newton Highlands (probably a little high for the stretch west of Brookline Village, but not too much), and 8-10 minute headways on each branch -- more than adequate for Needham, and workable for Riverside.