I'm guessing they mean that Back Bay would only be usable by Worcester/Framingham trains, and the Needham Line would run to Forest Hills, and that the Fairmount, Franklin, and Providence/Stoughton trains would shut down completely, but I'm not sure. (In theory, Franklin trains might be able to get to the Fairmount Line without interacting with the Providence Line that Amtrak controls, but then there's really no good transfer point if they can't get into South Station. I guess they could maybe run buses between South Station and Uphams Corner or something.)
It's not a theory, it is reality that Franklin line trains can get to the Fairmount line without touching the corridor. The connection is a bridge over the corridor. Even now under normal operations, at least one or two rush hour Franklin line trains actually operate over the Fairmount line, rather than the corridor. The only real question is, who controls the switch off of the Franklin line that determines if the train proceeds down to the corridor or across the bridge and onto the Fairmount line? If it's Amtrak, which seems unlikely but is not impossible, then they have a problem. If it's the T, then it poses less of a problem.
However, there is still a bit of problem, in that the Fairmount line can't handle the normal traffic load that it carries, plus the load from the Franklin line. And that's with an operational South Station. I'm not sure what they can handle if South Station and all of its tracks are off limits. After all, the Fairmount still feeds into the Old Colony corridor tracks, which in many places go down to a single track. That doesn't leave a whole lot of turning room, unless they send trains to the yard.
And that poses yet another question, can the T get it's trains out of the yard? I'm guessing that they can, but I'm not sure how they can get trains over to either the Needham or the Worcester line without crossing the corridor.
I'm also kind of surprised that Forest Hills would have a usable Commuter Rail platform that Amtrak doesn't control dispatching for.
Well the Forest Hills station is on the west side of the corridor, so it is possible that at that point the T controls the track closest to the Orange line from that platform out to the branch off of the Needham line. The track on the other side of the platform though, that being the one closest to the two electrified tracks, is controlled by Amtrak. That I know for sure based upon a problem on a Franklin line train that I was riding last month.
And I wonder if they really mean Braintree. I would have expected that if Amtrak only controls the parts of the system used by the Northeast Corridor trains, that Quincy Center and/or JFK/Umass would be useable; and the Greenbush line branches off a bit north of the Braintree station on the Red Line, so if the Globe was researching this carefully and writing precisely, the strike might completely disrupt service on the Greenbush line.
I don't believe that Amtrak controls that line, at least until it gets near the Amtrak and T yards, but I'm not positive.
And if the strike were to go on for long enough, is it possible that service on the north side of the system might eventually be disrupted by difficulties in getting locomotives and coaches to maintenance facilities?
I'm not sure just how things break down regarding the T's yards, but I do know that they have a massive repair shop just north of North station. One can see it within minutes of departing from North station on the left side of the tracks, just after crossing the river.