Here's a combined Blue/Green "rethreading" crayon that I've been working on for a bit. I'll go into specifics on the various parts below.
Blue is the simpler one to explain, so we'll start there.
Blue Southside:
-After the Red/Blue connector, Blue continues along the Esplanade to Kenmore (as has been discussed a bit here).
-From Kenmore, Blue swallows a (buried) C line all the way to Cleveland Circle, where it turns south to Reservoir
-From Reservoir, it takes over the outer end of the D Line, forking into Needham and Riverside branches
Blue Northside:
-From Wonderland, Blue hooks west under Revere St and Squire Rd
-Blue forks at the Squire Rd/Rte 1 rotary into two branches: South Lynnfield and Lynn (
technically making this a BLX crayon, haha)
-The Lynn branch goes underneath the Saugus bike path, the Lynn Common, and Market St/Lynnway to terminate at Lynn Beach.
-The South Lynnfield branch runs under/down the center (depends on budget) of Route 1, terminating at a traffic sponge P&R near the 128/1 interchange.
Green is a little more tricky, but in brief, there are now two trunks running four branches: an A/B trunk and a C/D trunk. The two trunks generally run independently of each other, but have opportunities for trains to move between them if necessary.
The destination pairs are:
A - Watertown to West Medford
B - Boston College to Medford Sq
C - Boston College (again) to Everett (Eastern Ave)
D - Forest Hills to Revere (Squire Rd)
More below, here's the overview map!
A little more detailed view of the Southside.
I decided to go with Blue taking C instead of D because Coolidge Corner is a really strong destination, and a near perfectly-straight alignment + stop consolidation + subway conversion = very quick and efficient service. Reservoir becomes a transfer station between Blue and Green (now the C line in this crayon).
As for Green:
The A/B trunk runs through Kenmore. The A Line branches off at Packard's corner, straight shot on/under N. Beacon to Watertown. The B Line is unchanged, maybe some stop consolidation.
The C/D trunk uses the Tremont - Marginal - Back Bay alignment. The tracks are buried under Huntington, then the C/D split happens at Mission Park. The C runs under Boylston St to an underground Brookline Village station, then ducks under the dog park to Brookline Hills. After Reservoir it cuts under Chestnut Hill Ave to join the B line tracks and terminate at Boston College.
Having both the B (from the A/B trunk) and the C (from the C/D trunk) terminate at the Boston College yard creates a handy opportunity for trainsets to switch between trunk lines as necessary with zero deadheading if you do it right. It also gives riders from that station their choice of which trunk line they want to use to get downtown.
As for the (new) D line, it hugs the Jamaicaway/Arborway all the way to Forest Hills. While it's not as densely populated at Centre St, it also doesn't overlap as much with the Orange Line walkshed.
Of course, downtown is where the fun really begins! So let's zoom in on that.
The A/B trunk approaches from Kenmore as normal, but then hooks northeast after Charles St, under the Parkman Bandstand. As best I can tell, there should be enough space under there for this since the garage is situated a bit farther to the north. This cuts Boylston and the famous screeeeeeech out of the equation for the A/B trunk. A bit of a loss, but Arlington is close enough for it to be okay.
Since C/D approaches from Tremont it still serves Boylston, without the screeching this time.
Park Street is where the fun begins: not only is it the Red Line transfer, but it's also where the A/B and C/D trunks can transfer between one another. The Central Subway has enough tracks to accommodate this, but squeezing two extra tracks under Tremont St to the north to accommodate a second trunk is unlikely to work. So this is where we play the "
crazy and stupid but maybe crazy and stupid enough to work" card: the A/B trunk will use the normal Park Street to Government Center tunnel, but the C/D trunk is going to do the same trip
as an elevated. So we're still quad-tracking, but two tracks above and two below.
Yes, this probably requires blowing up historic Park Street station and starting over. That's why we're in the Crazy thread.
Anyway, the A/B branch continues to Haymarket/North Station as normal, nothing to talk about there. The C/D runs as an elevated, then hooks east over Government Center and the steps between City Hall and Fanueil Hall. It cruises over Congress Street and North Street, then stops at "Fanueil Station," located above a pedestrianized Clinton St. The elevated slides over the Kennedy Greenway (this is also why I went with an el -- good luck trying to get any more tunnel perpendicular under the Big Dig) and then "lands" on Commercial Street, where it dips into a tunnel.
The C/D tunnels under Commercial Street, stopping at a North End station under Commercial @ Hanover. Then we tunnel under Langone Park and the Harbor to get to Constitution Wharf, then turn under Chelsea St to run parellel to the Tobin.
Phew! That was fun. Now the Northside bit.
Blue has been explained already, not much to add. A/B is relatively boring. A is just the Green Line Extention + West Medford. B departs from GLX to run underneath McGrath, then turns northwest under Mystic Ave all the way to Medford Square. As for Union Sq, that becomes part of another Urban Loop crayon that I'm not getting into right now.
C/D is a little more interesting: the tunnel dips under the Tobin at Medford St in Charlestown, serves a station at Barry Field, then tunnels down under the Mystic.The split happens underground around O'Malley Park in Chelsea.
The C heads north to the industrial park, where it emerges to the surface and runs along the existing freight ROW. From there we pop under Sweetster Circle and then it's just a straight shot up Broadway!
The D runs under Commandants Way, turns southeast under Williams St, then utilizes the JJ Mahoney Rest Are to partially mitigate the tiiight turn at Williams and Park St. Once we make the turn, though, we're in the clear for another straight shot up Broadway through Chelsea and Revere, terminating at a Blue/Green connection station at the Squire Rd rotary.
Anyway, plenty to nitpick and a bit of hand-waving to make this all work, but the end result is pretty nice: the Green Line goes from an unbalanced "3 + 1" system with a heavy Southside bias to a "2 + 2" system of 99% independent trunk lines that's equally balanced in the north and south. And all we had to do was blow up the country's most historic subway stations and spend a few billions to do it!