The important detail with this is that there are two platforms and four tracks at JFK/UMass, so a Braintree train could terminate and reverse there without affecting Ashmont trains at all. One track map (don’t know how accurate it is) suggests there are two crossovers between JFK/UMass and North Quincy so reversing Braintree trains here is pretty easy, up to the point of there being no signals and no automatic controls for the switches.