It's not going to be any sort of net-positive to the Blue Line to branch it. Branching depletes headways to destinations that formerly used to be mainline, and the parts of Eastie, Revere, and potentially Lynn that are served beyond the airport are growing too fast to be kneecapped like that.
This wouldn't be comparable at all to when the Red Line was branched to Quincy in 1971. Trains from Harvard to Ashmont ran 4-car max, and were frequently chopped back to 2-cars only off-peak. Headways tended to be throttled back well below line capacity off-peak as well. Ashmont has aggregately better transit now as a branch than it did 50 years ago when it was mainline because demand dictates the line can't doesn't run half-cocked at any time of day.
Demand on Blue and Orange have grown that way too. Blue's now 6 cars with expanded fleet, and Orange is getting expanded fleet to improve headways. Those mainlines are no longer going to run half-cocked, because demand end-to-end requires it. So introducing new branching becomes very destructive if it does anything to diffuse mainline-level service levels to these areas of growing demand. Airport spur is exactly what you DON'T want to do, because it sacks Eastie/Revere/(Lynn?) with service loss.
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Everybody else explained well enough why there can't be a transit line between South Station and Airport. The decision was made 25 years ago not to build a center transit bore with the Ted. It's now not possible with the connecting tunnels and whatnot on each end to go back and undo that mistake by adding an adjacent tunnel. Paths are now blocked. It's going to be SL1-only now and forever. Silver can share the Transitway with trolleys just fine, so the trolleys terminating at the Seaport and the airport buses terminating at South Station can always co-mingle just spiffy. But we have no means of replacing SL1 with a real rail line.
And we don't really need to, or spur Blue on the north side, because Urban Ring Phase II terminates at Logan station. A trolley can start its run at Silver Line Way, go inbound to SS, inbound to Boylston, outbound to North Station and Lechmere, then pick up the Urban Ring through Everett and Chelsea and terminate at Logan-Blue. Maybe on some future Massport renovation of Logan they do a grade-separated elevated busway around the terminals to speed up SL1. Then you can send that trolley straight through from Logan station to each terminal, co-mingled with SL1. Want SS...pick up the silver bus. Want NS, a Green one-seat, or transfer to Red or all the Green branches...pick up the Green trolley. Does everything you need it to from either side of the Logan-Downtown circuit.
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Blue to Salem is realistic, and an actual "Universe of Projects" listing in various MPO transit studies. The Eastern Route was built at 4-track width between the Lynn side of Saugus River and the Salem tunnel. Only a few of the post-1950's overpass renewals knocked out the #3 & 4 berths for simpler structures. Wonderland-Lynn actually costs more than Lynn-Salem because it involves a very expensive river crossing, jump-over from the Blue (ex- Boston, Revere Beach & Lynn RR) ROW to the Eastern ROW across wetlands, and a segment of the Eastern that was always a 2-track causeway. But once you're at Riverworks it's pretty much free and clear all the way up.
So priority should be getting to Lynn (and Charles MGH) in the first place, dealing with that sticker shock, and making sure that sticker shock stays in-check without needless bloat. But once you make it to Lynn in one piece, going further is straightforward. Start the Salem studies, build up the coalition...and when the time comes, it should be a relatively low-drama affair. Work mainly involves:
- Quadding back up the bridges that were cut back to 2 tracks in more recent history.
- Embankment and retaining wall work. Some areas got additional dirt pack over the past century to shore up hillsides and whatnot, and lost a little bit of width that needs to be re-scooped and re-stabilized.
- Reconfig Swampscott station. Station house is a historic landmark, and it's densely abutted. May need 1 or 2 buildings of property taking next to the station parking lots and some artful angling of the 4 tracks over Burrill St. to avoid clipping the adjacent buildings. All doable since rapid transit would be swallowing the CR station here, but this is maybe the tightest fit on the extension.
- EIS'ing the swamp causeway north of Swampscott station. This segment was originally graded for quad-track way back in the mid-19th century, but it never actually carried 4 tracks. It's easily wide enough for 3 tracks because of the adjacent power lines and telltale extra ballast space next to the outbound track. So it would need to be rounded up 150 years to current environmental regs for how much buffer is needed between track and vegetation (which has encroached the original grading). Probably means some shaving of rock outcrops to create level space for all 4 tracks. As EIS'ing goes this isn't scary at all, but you pay the going rate for protecting a swamp.
- Storage yard. That would be some chunk of Castle Hill freight yard currently buried in weeds a few blocks south of the tunnel. Depending on what side of the ROW Blue runs on may have to do a small flyover to hop over commuter rail here.
- Can't go through the tunnel; it can't be widened enough for 2 rapid transit tracks next to 1 RR track. So climb up the side wall and stub-out at Mill St. in the DPW yard next to the police station. End of the line.
Stations past Lynn can be:
- Swampscott -- displaces CR station
- Hawthorne (optional)-- @ Essex St./Danvers Rd. Access to Swampscott Mall, 455 & 459 buses, area resididential and employment, crossroads between Routes 107 & 1A. Spacer for over-long distance between Swampscott and Salem State stations. Omit if ridership projections underwhelm.
- Salem State University -- @ Jefferson Ave., same place infill CR station has been proposed (Google "North Shore Transit Improvements Study" on the T's website and there's renders of it). Blue displaces CR station. Walking distance to Mass General Children's.
- Salem -- @ police station on Mill St. Same location (above, instead of in the tunnel incline cut) as pre-1988 CR station. Probably worth relocating the CR station back south-of-portal into its old cut for a unified superstation. That ghastly parking sink to the north is so anachronistic the town will be sick enough of the park-and-riders in a couple decades to want to redev that thing into something more downtown-y. I suppose if you need a lot at the new station you can plunk it at the north tip of the Castle Hill yard next to the electrical substation.
Remaining CR stations south of Beverly would be
Chelsea,
Lynn (huge bus terminal),
Salem (sizeable bus node, some reverse commutes, last possible place to transfer to/from all 3 CR branches before Peabody diverges). Riverworks would already be swallowed by Blue on the Lynn extension, Swampscott and Salem State U. by Salem extension. Expect CR ridership at Lynn and Salem to resemble Malden Ctr.: pronounced skew to alightings over boardings, primarily a bus transferee audience instead of general commuter audience. i.e. "significant niche" patronage.
