The whole of the Eastern Route was a 4-track RR. Assuming the SL Chelsea busway gets built as designed to Mystic Mall, continuing this circuit past Sullivan to the Mall only leaves three critical decisions to make:
1) Crossing over/under from the west side of the Orange Line tracks to the east/Mystic side around Assembly. Those 2 freight tracks at Sullivan fizz out in the overgrowth
here...on Assembly Sq. Dr. opposite side of the tracks from those landmark "big tits" buildings. That's an easy one to engineer...quick flyover or quick duck-under. But do you do it at bus dimensions or LRT? And how close do you attempt to get to Assembly Station before having to hang a right towards the bridge? (NOTE: There are not many satisfying answers to the last question, so be prepared to compromise.)
2) The bridge. Where, how wide, how difficult an EIS'ing? How do your requirements for the placement of Assembly station impact the bridge trajectory? (NOTE: If you haven't been ready to compromise yet...you will now.)
3) Crossing to the other side of the Eastern Route tracks. You start on the north/Gateway Ctr. side of it because of the Everett Terminal freight tracks. Then somewhere between 2nd St. and the Mall you have to duck under to the south side to meet Chelsea station and the busway. Where do you do it, and does where you choose to do it require any land acquisition? Do you do it at BRT or LRT width?
Path of least resistance is probably:
1) Don't get too cutesy about Assembly because trying to get too close to the OL platforms overcomplicates the bridge. Get within about 500 ft. of the station then fly over/under the Orange tracks. Touch down in the park and put a simple platform there. Make a covered connecting walkway to the OL stations and egress to all the Assembly attractions. It's shorter length than the Wellington garage walkway by a couple hundred feet, so don't feel like it's necessary to distort the build to deliver Assemblyites door-to-door in a covered chariot. (Satisfying?...no. Necessary...yes. Good enough...absolutely.)
2) Build a new bridge for the Eastern Route on its
old alignment to the pre-1988 drawbridge, about 100 ft. south of the current bridge. The footprints of the old bridge are clearly visible on both sides of the river. It would be a shorter span than the current one because it hits terra firma on the Everett side sooner while the current bridge stays in the marsh for a few hundred more feet. Move earth to build up the embankment grades, build the span same general height as the current bridge. Shift the tracks over. Then use the old bridge for rapid transit. LRT, since we know that doesn't require any mods to the bridge width or erecting tall highway guardrails. This more or less decides for you how close you can swing to Assembly, but it's the cheapest and most straightforward way to get it done. (You will now feel at peace with your "good enough" decision about Assembly station placement in #1 looking at what a boondoggle this bridge would be any other way.)
3) Switch sides
hereabouts. 3rd Ave. is a useless grade crossing that deserves to be outright blocked off and closed today, so nobody cares about the road. Parking lot on the north side on 3rd is a good enough place to portal-down, grass on the south side other side of 3rd is good enough place to portal-up on approach to the upcoming Mall stop. Closed-off 3rd St. itself ends up being three-quarters of the required space, and it avoids the stream to the west. Again, much easier to do in a compact space on an LRT fixed guideway than with BRT turning radius.
New stations north of Sullivan and west of Mystic Mall/Silver Line:
-- Assembly. Or close enough in the park with the sheltered walkway to the OL and street level.
--
Everett Casino/Gateway Ctr. Overhead walkway (or roadway) connects the Gateway and casino together approx. where that official-use-only grade crossing is. Ramp up from the platform.
--
Santilli Circle. Walk straight off the train onto the Saugus Branch ROW trail to the neighborhood. Paths connecting to Paris St. east and footbridge up to the Broadway overpass. Maybe center the station closer to Broadway.
I don't really think any of these need to be prepayment stations. Unless the casino outright pays for it. So unless that's now some sacred commandment these don't--and shouldn't--need to be full-staffed or full-enclosed when effective proof-of-payment technology exists.
Once you touch down at the Mall you're on the Silver Line busway. Theoretically you could throw rails-in-pavement and do both co-mingled east of the Mall...but let's assume we don't want any Frankenstein stuff and it flips to LRT-only. Assume the 2nd St., Everett Ave., and Spruce St. grade crossings have all been eliminated by this point with road bridges leaving light-traffic 6th/Arlington underneath Route 1 as the only uneliminable. There's the rest of your Ring ROW to the Chelsea St. bridge, no-drama.
Scoot street-running across the bridge. Movable bridges are a non-challenge for trolleys. Turn out onto the Eastie Haul Rd. being built now on the RR ROW...same one the Silver Line is going to use. 2 choices:
-- (1) The cheap one: rails-in-pavement, street-running. It's restricted to truck and transit traffic, so this is not a traffic problem for a GL branch that has branchline headways.
-- (2) The expensive one: Scoop out the earthen embankment along Brennan St. to re-widen the ROW to its former width, widen 2 overhead bridges, and shift the haul road a few feet to the side so there's grade separation. This cut used to be exactly as wide as Track 61 + Southie Haul Rd. was before it gradually got filled in, and the Haul Rd. construction isn't scooping out all of the fill. It's an expensive do-over of the retaining walls and a few blocks of haul road to get the grade separation, but yes it is feasible.
-- (1), then (2) some other day. Do it the cheap way rails-in-pavement, then when you string the rest of the Ring together and crank the service density up another few notches go back and finish the grade separation. The whole Ring ain't gonna be built at once. If this is the first piece of it to go online it might be another 1-2 decades before Cambridge is shoveling riders into Brickbottom Jct. off the Grand Junction. You've got time to breathe here.
I feel like that could have some pretty sweet veiws of the city and would be profitable