they already run this route. SLW.real_EthanHunt -- The key is adequate frequency
So how do we insure adequate frequency -- make sure no bus is delayed by the effects of surface vehicular traffic -- in other words tunnel under D St -- so that some set of the buses [i.e. SL0] can loop from South Station to Silver Line Way entirely underground. The other Silver Line buses would be metered into and out of the stream @ Silver Line Way in such a manner as to not disturb the high frequency service [SL0]
T under D is a waste of money. you can accomplish the same benefits for the buses by installing bus priority signals to cross D St.