The dual-modes that currently exist pick up traction-motor voltage, not catenary voltage. Could catenary-voltage dual-modes be made? Probably. Could another Somewhat-Big Dig be saddle-bagged on the Really-Big Dig to provide cross-Boston rail service? Technically, possibly. Fiscally or politically? Exceptionally improbably, for a least another ten years until post-Big Dig amnesia can set in.
Besides being named after compass points, what else do North and South Stations have in common? Just before each station's stub-end platforms are restrictive accesses. The designing of an approach from the eastbound curve before South Station to an underground tunnel seems, at best, difficult and also would seem to need to bypass the platforms. The outflow of the Charles is bridged just beyond North Station, and that would seem to require surfacing somewhere near the Gilmore Bridge. Couple those issues with providing platform access for passengers, and architects and engineers on such a project will prematurely become what I already am -- aging and balding. North Station could be the passenger-access by doing a reverse move once the equipment had surfaced, but this is starting to get as confusing as the old Pennsy in old Philadelphia.
"A gray crossover is definitely not company transportation."