A couple months back I spent a lot of time finally digging in to one of the thorniest parts of the Green Line Reconfiguration: the Aldgate junction proposed for Brickbottom Junction. For some reason, I never actually got around to posting what I came up with. So, I'll look back at my notes, and see what we can do here...
There are 4 journeys Brickbottom Junction needs to support in order for all this to "work":
- Lechmere <> Medford
- Lechmere <> Union
- Cambridge (Grand Junction) <> Sullivan
- Cambridge (Grand Junction) <> Lechmere
(That last one is potentially optional, if you can find someplace for a Cambridge <> Sullivan LRT line to have a transfer to either/both of the Medford or Union Branches. I'm skeptical that's doable, and I think there is merit to a North Station <> Lechmere <> Kendall route.)
(And yes, I am excluding Lechmere <> Sullivan; as I've detailed elsewhere, I don't believe that this route is the best use of resources, and while the Lechmere -> Sullivan piece is very easy to build, the Sullivan -> Lechmere piece will be nasty, and either be inordinately inexpensive [tunneling or super-elevateds] or introduce bottlenecks [flat junctions].)
1. Lechmere <> Medford and 2. Lechmere <> Union
These are handled by the existing GLX infrastructure, which should require little modification.
4. Cambridge (Grand Junction) <> Lechmere
, you reclaim some/all of the Somerville Avenue Extension road alongside the ROW
as it crosses under McGrath Highway, branching off the Union Branch at the corner of the Brickbottom Artists' Association building, and start diving down below grade level (shown in dotted green here). I'm not sure of the exact depth an underpass would need to be in order to safely cross under the Union Branch and Fitchburg Line; that said, a 6% grade reaches a depth of 20 feet in 333 feet of running distance, which just about fits in under the McGrath overpass. Once underground, there should be enough space for a gentle curve (i.e. 80' radius or better) that avoids the foundation of the nearby building before tunneling under and then surfacing within the Grand Junction ROW.
If 20 feet isn't deep enough for an underpass, we can achieve 660 feet of running space to enable a 40 foot depth by cutting under the McGrath Highway embankment to swing wide into a question mark shape. Even with 85'-radius curves, it should be possible to avoid the building foundations. See the route in red (shown in solid color, but would be a tunnel for most/all of its length):
, we are basically going to clone the Union -> Viaduct incline and put it south of the mainline tracks. Now, this will require a little bit of rearrangement. There are currently three
mainline tracks between McGrath and the Viaduct (let's call them North, Middle, South):
Vanshnookenraggen's track map
is helpful for illustrating what's going on here; Middle and South are the through tracks, while North is a stub-end:
And a photo I took from an inbound train on the Viaduct also helps, somewhat:
As can somewhat be seen in the above photo and more clearly visible in the StreetView
, there is available space next to the South track (possibly with some relocated support pillars). But, as the ROW continues east, it becomes constrained by the buildings at the eastern corner of the lot (as visible in the photo).
So I propose using the wiggle room available underneath the McGrath overpass to shift the mainline tracks "up" one "slot".
This opens up the slot of the South track to create the new incline, and join the Viaduct just south of the current merge (new incline shown as a dashed line):
Note that you still will need to fit the new incline under
the Community Path. I think
this will be doable, though it may be necessary to relocate the Viaduct merge further south than I've shown it here (if you need to duck lower than I've estimated to clear under the Path, in which case you need additional running length to rise up to meet the Viaduct, which should still be doable).
You'll also need to do a little bit of rearranging of the mainline tracks on the eastern side of the Viaduct, including weaving through some support beams, but it shouldn't be anything super aggressive. In the diagrams, light purple is used for the non-revenue mainline tracks, mid-purple is the typical revenue mainline tracks, and dark purple is new track that would need to be built to reconnect things (and the light purple fill is the yard, where I didn't draw out all the tracks).
(Spoiler alert, this screenshot also includes new LRT tracks to support Grand Junction <> Sullivan, but I'll get to that.)
At this point, you now have a grade-separate flying junction that supports Lechmere <> Grand Junction service without creating crossing conflicts with other services.
3. Cambridge (Grand Junction) <> Sullivan
Half of this one is very easy, and half of it is very much not. Let's start with the easy one.
this part of the junction is already built, in the form of the yard lead running from the GLMF to the Union Branch. With some modifications to the yard tracks, you have your revenue tracks connecting Sullivan to the Grand Junction (via the underpass discussed above) right there.
If you want to go all out (or include in a subsequent enhancement build), you could look at adding a parallel track north of the current track, connecting the current yard lead to the diverging track at McGrath, so that Sullivan -> Cambridge trains don't need to interfere with Lechmere -> Union trains. The problem here is that you would be right up against the back walls of the adjacent buildings, and I'm not sure how close you can actually put an active RoW against a building like that. The parallel track is valuable, as this stretch will otherwise see all:
- Lechmere -> Union
- Lechmere -> Cambridge
- Sullivan -> Cambridge
trains, so this stretch could become a bottleneck. But I think it may still be workable.
is where things get hairy. Coming from Cambridge, this track needs to cross over or under:
- McGrath Hwy
- The Cambridge -> Lechmere incline
- The Fitchburg Line
- The Community Path
- The BET yard leads
- The Union -> Lechmere incline
- The Viaduct
- The Lechmere -> Union incline
- A potential future Lechmere -> GLMF yard lead incline
Running flat at-grade will be a non-starter, unless you plan to sink the Fitchburg Line tracks, which will be both cost- and space-prohibitive. Running elevated looks more possible at first, but would need to weave under
the Community Path, over
the Viaduct, over
the Lechmere -> Union incline, and then somehow interface with the Lechmere -> GLMF incline (probably needing to pass over it altogether). Alternatively, one could "go wide", remaining at surface level south of the Fitchburg Line from one side of the junction to another before rising into a separate viaduct over the BET. In both cases, however, I think
that you can't make the necessary height changes within the short distances while still maintaining a 6% grade.
So, it's a tunnel. A short tunnel, but a tunnel, taking what I think is the most direct (shortest) route, but which also happens to cut right through the heart of Brickbottom Junction. Hopefully it could be dug in stages to reduce disruption to Fitchburg service, but it definitely will not be pretty. On the other side, it surfaces to join one of the tracks repurposed from the GLMF Yard to continue to Sullivan. (In the diagram below, the dashed blue line indicates a surface track that could connect to a Lechmere -> GLMF incline, if ever built.)
If memory serves from when I first sketched this out, the paths I've drawn should
give enough running distance to drop 20 feet below surface by the time the tunnel hits the mainline tracks. On the eastern end, there is plenty of space to lengthen that if needed. It's a bit trickier on the western end, but I think it should be possible, if needed, to relocate the Cambridge -> Lechmere/Sullivan divergence point further west in order to provide additional distance to fit a drop to a greater depth than 20 feet.