from https://archboston.com/community/threads/crazy-transit-pitches.3664/page-276#post-464353
Recently someone asked about possibilities that get opened up if the LRT fleet is split — instead of finding one model that fits all needs, use multiple models for different lines. In general, my initial thought is that this does not enough benefit to outweigh the costs; separate fleets reduces operational flexibility and probably increases maintenance costs.
But, as a thought exercise, I want to game out the ruling radii if the existing network were split into “modern” and “legacy” systems. Building off the approach in the Green Line Reconfiguration thread, let’s say that the “legacy” system is:
- B Line
- C Line
- “Fenway Branch” (Kenmore <> Brookline Village)
- Boylston Street Subway
- Park <> Boylston inner tracks
And the “modern” system is everything else, with additional modifications including:
- Extended Huntington Subway
- Elimination of Heath St Loop
- D <> E Connector
- Reroute of Huntington Subway to Pleasant Street Portal
I highly recommend this master’s thesis from MIT: Strategies for meeting future capacity needs on the light rail MBTA Green Line In particular, Appendix G (pg 209 of the PDF) provides a thorough overview of all of the constraining curves on the Green Line, as well as prospects for remediating them.
In terms of “ruling radii” in a “Crazy Transit Pitches” future:
- Brattle Loop, 50 feet: the curve probably can’t be widened, but potentially something clever could be done to convert the loop into a stub-end terminal, and avoid the sharp curve altogether
- Park St Inner Loop, 49 feet: difficult to widen, but could potentially be removed or reworked into a set of parallel tracks that could still enable turnbacks
- Kenmore Loop, 55 feet: I have yet to see any convincing proposal for widening this one
- Lake Street Yard, 50 feet, 49 feet and 45 feet: if revenue terminating ops can be relocated to the median of Comm Ave, then I think flexibility opens up to rework this plot of land as needed
- Reservoir Yard, 50 feet: the curves going in to Prendergast Ave are the limiting factor here; they could potentially be expanded a bit, but not much
- Heath Street Loop, 50 feet: a number of solutions, including conversion to stub-end
The thesis linked above suggests that a reasonable target could be 60-feet radii for revenue tracks and 50-feet radii for non-revenue, which I agree with.
So, for the “legacy” system, the ruling radius would probably remain 50-foot non-rev/60-foot rev — maybe could be raised to 55-foot non-rev if Lake St and Reservoir can be cleaned up. On the other hand, perhaps you could keep the Park St loop as-is and live with the “legacy” fleet just using a 45- or 49-foot radius — it would still require custom vehicles, but perhaps with a more modest set of customizations.
For the “modern” system, I think the Gov’t Center <> Haymarket stretch still poses the most challenges. If we abandon the Brattle Loop (or otherwise reconfigure some sort of stub-end terminal there to short-turn northside trains), we are stuck with the 60-foot radius going southbound into GC. If that can be widened, and a few small curves in the Riverside and Reservoir yards can be modified, then you could have a “modern” system-wide 66-foot minimum radius.
Hmm.
The “legacy” system would have a total length of 10 miles (including the Fenway Branch and excluding an A Line, which happen to be broadly equal length). (An additional 2.5 miles for the Mattapan Line, but that should be covered by the Type 9s for a generation, so can be excluded from this discussion.) The “modern” system would start at about 17 miles with the existing components, with potential additions of:
- Needham, 3.75 miles
- Porter, 1.5 miles
- Nubian, 2 miles
- Grand Junction (Cambridge), 2.25 miles
- Sullivan, 1 mile
- Sullivan <> Chelsea, 3 miles
- Watertown, 4.5 miles
- Waltham/128, 8 miles
(System length isn’t the perfect proxy for rolling stock needs, but I’m just using it for a rough estimate here.)
The existing system of 27 miles uses about 200 vehicles, which I believe is supposed to be handled by the order of the Type 10s, which will necessarily need to be usable on both the legacy and modern systems. Assuming at least some of the above extensions are built within the lifetime of the Type 10s, at some point there will be need to be another order — so, the question becomes, at what point could/should subsequent orders be “modern-only”?
Let’s assume 10/27, or 37%, of the Type 10 fleet gets devoted full-time to the legacy system. The modern system, with each progressing expansion, would increase in length from 17 miles to:
- Needham, 21.75 miles, 128% of current
- Porter, 23.25 miles, 136%
- Nubian, 25.25 miles, 149%
- Grand Junction (Cambridge), 27.5 miles, 162%
- Sullivan, 28.5 miles, 168%
- Sullivan <> Chelsea, 31.5 miles, 185%
- Watertown, 36 miles, 211%
- Waltham/128, 44 miles, 259%
Making totally unqualified wild guesses, the existing fleet could probably be “stretched” to handle extensions to Porter and Needham, especially with some initial frequency adjustments on the Needham branch. But once you add on the Nubian Branch, or any of the “Urban Ring” services, I think you’re looking at another order of vehicles.
So… could the “Type 11” fleet be modern only? I think so. In terms of the modern system, really the only “problem spots” that would remain in use from the current system are the situations at Government Center. Everything else would either be new builds, or utilizing existing tracks with good radii. So, assuming isolated ops, the modern system could be moved off of custom-builds relatively early on.
Could an initial small Type 11 order be useful even without interoperability on the “legacy network”? Again, I think so. Assuming the T11s go into service at the same time as a Nubian branch, the modern system will be about 25 miles long, while the legacy system will only be 10 miles. By my math, a small order of T11s sufficient to “top off” the LRT fleet to levels roughly equal to today would have the T11s comprising about 27% of the overall fleet, leaving 73% of the fleet available and flexible to serve the 10 out of 35 miles (28%) that would require “special” cars. The modern system would be so much larger than the legacy system that the T11s would probably always go to the modern system anyway. (And there wouldn’t actually be that many opportunities for vehicles to swap from one network to the other, since they would be largely physically separate by design.)
(And I think that the T currently operates under a restriction broadly similar to this: 13% of vehicles cannot serve ~19% of the system, due to Type 9s not being usable on the E Line.)
I think overall this boils down to, “The Type 10s should support both the modern and legacy systems, but the next order of vehicles — the hypothetical ‘Type 11’ — could/should potentially be built to handle only the modern system, with a 66-foot ruling radius, assuming that the order is being made due to system expansion, and not due to replacing the Type 10s.”
The only additional wrinkles I see are the potential “new” street-running builds; branches to Nubian, Allston/Harvard, and cross-Longwood would be done largely outside of existing ROWs and in more direct interface with the open environment. I haven’t looked through those alignments carefully enough to be sure that a 66-foot radius is possible on all of them. That said, alignments in those locations would need to avoid excessively sharp turns anyway in order to prevent wheel squeal. So, hopefully, the one problem would solve the other.
Finally, for completeness’s sake, these days I mostly am thinking about non-interlined Urban Ring services. So, if those get built first, then arguably you could justify a non-legacy Type 11 build that much sooner.