How is this any different than say the sharp curve approaching Harvard? Or any of the sharp Green Line curves, IIRC around Prudential, the merve into the central subway, Boylston? Oh, the Bowdoin loop is single tracked. So is the E train under Mass Ave. And how do you get out of Porter in case of emergency??? You're single tracked, and 100 feet underground.
I still don't think the Red/Blue connection is entirely justified. The entire system hinges on transfers downtown, or your only other option is the somewhat serviceable Crosstown bus routes. Or you can get off at Bowdoin and walk to Charles/MGH, as it will take you the same time as transferring to the Orange or Green Line. The drawback is the six months known as New England winter. But if you're going to punch the Blue Line down Charles Street to a subterranean station connecting with the Charles/MGH headhouse, then you mind as well tunnel under the Charles and continue it on as the never built inner loop.