Tunnels are technically engineered for 100-year lifespans, but that assumes something like a RR tunnel that's going to get little, if any maintenance, and is still out exposed to the elements instead of below-street. Something like the Hoosac Tunnel out in Western MA, or the Cascades Tunnel out west. Hoosac just got its first serious century rehab last decade when they adjusted the clearances to handle autorack freight. T tunnels get nightly maintenance and structurally are in perfect shape. More perfect than the Big Dig. Probably the worst-condition of them all is the Alewife extension at Porter--built 1981--and between Alewife and Davis--built 1984--because they won't fix the @#$% water leaks that've been gushing for 25 years. I don't think they've ever had to do any concrete or steel support work on the Central Subway. I don't think it's a stretch to say the subway tunnels structurally can last 200 years without anything you'd consider a major do-over. I mean, the aqueducts feeding Boston all its water are up to 150 years old and in perfect working shape. There's aqueducts and catacombs in Europe that've been in continuous service for 2000 years without much maintenance.
It's gonna be incredibly difficult to expand subway dimensions downtown. For the same reason Silver Line Phase III proved unbuildable and the Big Dig so expensive. Too many old building foundations, too thick a net of undocumented buried utilities, too funky a water table with the Back Bay landfill, and too much glacial debris, archaeological crap, and toxic trash buried in the soil. That includes building under-tunnels on existing lines, which you really can't do without stable bedrock or the kind of space-age tunnel jacking they did on the Big Dig where it passes under the RL. The only new subways that are practically buildable in Boston are ones that are under existing ROW's, like the North-South Link in the cleared space under the Big Dig and its portal approaches all under RR tracks, or burying the B and E lines. Or doing short, not-so-painful infill jobs like the Red-Blue Connector, the street-running portion of Huntington Ave., or reactivating the Tremont St. tunnel and bringing it the extra couple blocks down under the Pike to feed trolleys onto Washington St. In those cases you're respectively outside of the Back Bay landfill under bedrock, outside of downtown proper, or under modern urban renewal land that got bulldozed for NEMC and the Pike in the mid-60's.
Only one they really have to take a deadly serious look at is the 2-track pinch point on the Green Line between Park St. and GC. That one most likely can't be expanded to 4-track because of how narrow Tremont St. is and the burying ground. But if they could even squeeze 3 tracks there it'd absolutely be worth going for even if that means you're 2 tracks outbound but 1 track inbound (or vice versa).
For Washington St. tunnel relief they really gotta look back to the past and bringing the equivalent of the Atlantic Ave. El back through the North-South Link. Doesn't necessarily have to be Orange Line...I think a whole independent subway line strung together from the Fairmount Line, N-S Link, and Green Line Somerville running on unmodified heavy-rail Red Line cars (potentially interchangeable with the RL from Columbia Jct. via the Cabot Yard leads) would fill that function nicely. And there's a lot they can do in the meantime to pack a lot more capacity on all 3 heavy rail lines like next-generation signaling to get headways from 4 to 2 minutes and doing the Blue Line extensions and Urban Ring so Orange, Red, and the Central Subway don't have to carry such a skewed load while Blue is virtually empty and there's zilch moving radially. So I think we're 50 years away from having to ponder 4-tracking downtown on anything but Green because of the options in play and ESPECIALLY the N-S Link option restoring a rapid-transit routing that wasn't viable in 1938 when service ended but would definitely be viable in 2028.
