@Teban54, thank you for your thoughtful comments and kind words! I'm sorry it's taken me so long to respond.
Overall, I should clarify that these cost estimates are extremely provisional. If we're thinking in terms of "
significant figures", this estimate probably has only one sig fig, meaning the conclusion here is that the cost is 1 x 10^10 -- something akin to "ten billion, give or take ten billion", or, more practically, "between 5 and 20 billion". There's a lot of variability and a lot of uncertainty, and lots to debate (as we'll see below).
But overall I don't think that the variability and debate fundamentally changes the "between 5 and 20 billion" number, and I think the same is true for the NSRL -- which is why I think these projects are comparable and in fact
should be compared.
Riverside said:
For example, the Commonwealth Subway would be highly valuable for increasing speed and reliability, but isn’t critical to any particular project.
I actually think that in a full-build GLR, the Commonwealth Subway will be more important than shown here. Depending on build configuration, up to 4 branches may traverse the stretch at what's now BU Central: Boston College, Oak Square, Harvard, and Grand Junction. That's the same number of branches as present-day Copley-GC (which is already a bottleneck on today's Green Line), but even worse, running at street level for 0.7 miles with no grade separation and possibly no signal priority.
To make things worse, each of the 4 branches will likely involve some street-running, with the possible exception of Harvard. Especially BC/Oak Sq, and especially if Grand Junction service originates all the way from Chelsea or Airport. That's a huge reliability concern that may be even worse than today's Green Line (since the D has full grade separation).
You raise compelling points about how "beefy" of a Comm Ave subway is needed to make this whole thing work. I would argue we can simplify things a bit, though:
Up to 4 branches may traverse the stretch [between Kenmore and BU Bridge]: Boston College, Oak Square, Harvard, and Grand Junction.
I think all four of these branches being routed over this stretch is both unlikely and undesirable.
Sending a Grand Junction branch to Kenmore solves no problems and creates new ones: Kenmore station cannot terminate trains from Comm Ave without a significant/total rebuild, and is not itself an employment or residential destination -- both BU and Longwood are much larger hubs. Sending a Grand Junction branch around beyond Kenmore consumes capacity in the Boylston St Subway, and provides poorer Cambridge <> Back Bay and Cambridge <> Downtown service compared to a Grand Junction/Lechmere wraparound, the Red Line, or something via Mass Ave. (Mass Ave @ Grand Junction <> Copley is 1.5 miles via Mass Ave + Green, and is nearly twice that via BU Bridge.) If we can find a way to send a Grand Junction line into the Beacon St subway to access the Kenmore Loop that way, then I think it's fine. But I don't think it's worth spending significant money to enable a Grand Junction/Comm Ave wraparound.
A Harvard branch has a few different potential purposes:
- Urban Ring service between Harvard and something in the southwest quadrant (ideally Longwood)
- Urban Ring service between Harvard and West Station (for Porter-style transfers)
- Radial service from West Station to Back Bay/Downtown
- Radial service from Allston to Downtown
The Urban Ring services can't terminate at Kenmore, so a Comm Ave subway is less useful to them. Radial service from West Station is valuable, but could be achieved with strong Regional Rail frequencies (and in the long term I'd argue would be a strong western anchor for Blue Line service). That leaves a OSR between Lower Allston and Downtown as the unique benefit of running a Harvard branch via a Comm Ave subway. That's a worthwhile benefit, but I think it probably is comparable to the benefits of a restored Oak Sq branch, so I think we'd see either an Oak Square branch via a Comm Ave subway, or a Harvard branch, but not both.
Which is why I think it's more likely that we will see a maximum of two, not four, services between Kenmore and BU Bridge. If one of those branches is a greenfield LRT line to Harvard (especially a grade-separated one), then that might tip the scales in favor of a subway, but I don’t think it’s a sure thing.
Overall, the potential Grand Junction and Harvard branches benefit much more from a connection toward Longwood rather than Kenmore. If Fenway station becomes the “landing target”, then we’re also talking about a slightly shorter corridor as well — about 2600 feet vs about 3600 feet to Kenmore. So I think we get more “bang for buck” by focusing on building that connection, rather than spending the money on a Comm Ave subway whose services still will be impacted by street-running reliability. The Kenmore Division is always going to mix poorly with anything new we build, and I think that holds true here.
I agree that a 4 track subway can mitigate that, but where will those extra two tracks go after Comm Ave? I suppose you can build a new Kenmore Under terminal, but again, that's a lot of money to spend for a service that isn't even where people need to go. If we want to build a Comm Ave subway, I'd rather build it as an extension of the Blue Line to a short-term terminal at BU Bridge, staged to eventually extend to West Station. That would bring both the Urban Ring transfer and the Blue Line terminal to where the employment center actually is: Boston University.
Another consideration here is the BU Bridge junction. You need the junction to enable services to Packards Corner, West Station and Grand Junction. A surface intersection at the current BU Bridge junction doesn't make the cut to me given the existing traffic patterns, and a viaduct may have difficulty descending onto the BU Bridge ROW. So you need a tunnel for the junction even in a smaller build - and if that's the case, it may be an additional argument for just building the full Commonwealth Subway once, instead of in two stages (and disrupting traffic and B branch operations twice).
As I've alluded to above, I think the Aldgate junction at BU Bridge only needs to point in three directions: Cambridgeport, West Station, and one direction south (e.g. Park Dr). Packards Corner doesn't need service from anywhere other than Kenmore, and as I've argued above, I think Grand Junction <> Kenmore is a poor investment. (A non-revenue connection could be operationally valuable, although I don't think it would be make-or-break.) In some ways, I'd rather settle on a design for the Urban Ring stuff
first, and then add a Comm Ave subway as a secondary priority.
But yeah, either way, the Comm Ave subway is "only" $1.4B, which is practically a rounding error in this exercise. I definitely think it's an important question to resolve, but there are many important questions to resolve for the Green Line Reconfiguration overall, and I'm not trying to suggest I have all the answers so far!
I'm assuming you're using a street-running route via Mountfort St and Park Dr. It can work in a minimum build, but I'm concerned about reliability, given traffic, several signalized intersections, and the low likelihood of getting any dedicated ROW. This is in addition to adding even more complexity to the BU Bridge junction - now you have two possible directions on the southeast side.
I was trying to leave this corridor open-ended on exact route/alignment, because I agree there are problems with using Park Dr, though probably I should've erred on the side of more expensive estimates, and treated it like a "tunnel" segment. At "tunnel" costs (which probably are also a vaguely decent estimate for elevated costs), it's a little less than $1B. But, as mentioned above, to me it'd be a no-brainer to reallocate those Comm Ave subway costs to build a connector like this.
(And, assuming we can leverage Mass Pike air rights to avoid actually running at-grade along Mountfort, I
don't actually think we need to be quite so pessimistic about Park Dr. Close the northernmost block [between Mountfort and Buswell] to public traffic, reclaim the western half of the street between Buswell and Fenway station [easier said than done, true], and traverse one traffic light. That's not ideal, but that doesn't seem impossible to me.)