Re SWC: It's Boston-centric because it's about Boston: In the 1940s-70s highway plans there was the Western Expressway (now the Mass Pike Extension inside 128); the Northwest Expressway (now US 3 north of 128; I don't think it goes in any further as limited access, does it?), the Northern Expressway (now I-93 north of the city), the Southeast Expressway (now I-93 south of the Artery), and the Southwest Expressway (8 lanes of traffic with rail and subway in the median, Boston's own Dan Ryan Expwy. When the Southwest Expressway was cancelled with a lot of the land already taken and cleared (1973) they took the name for the rail-and-park combination. So the NEC comes into Boston through the SWC the same way it used to come in over the Boston and Providence or the New Haven (pick your era) and the same way the NEC goes into New York over the Hell Gate Bridge.
And I agree that change-at-Forest-Hills would be a non-starter. The Roslindale folks would find it quicker to take one of the constant stream of buses to FH; the Bellevue-Highland-West Roxbury folks might, too, since the buses on or near Centre St. are two or three times as frequent as trains even at rush hour, IIRC. The further out people wouldn't have those buses, but they could still find their trip fifteen mins longer or so (depending on where they wanted to get off in town). It's a good little middle-suburban express service and the people who use it have a right to want to keep it. OL conversion maybe OK, but still there would need to be something for the folks in N Ctr and N Hts.