Vertical clearance for Chelsea Creek
Back to the original topic of a Chelsea Creek crossing for any mode (cable car, LRT, etc).
Unless I'm mistaken, I think the limiting factor along the river may not actually be the
Chelsea St bridge (175'), but actually the McArdle bridge further west.
Wikipedia suggests McArdle only has a clearance of 157 ft when open, though I'm not sure about its accuracy due to it being a bascule bridge. So while the contradiction between the 175' Chelase St clearance and 170' Logan restriction is interesting, at the moment I'm not too worried with a bridge that doesn't reach the full 175'.
However, I am more concerned about
neighborhood impacts of such a bridge for LRT. The Tobin Bridge, which has
a clearance of only 135 ft, already has significant impacts to Charlestown an Chelsea, basically segregating Navy Yard and Admiral's Hill from the rest of the neighborhoods. From the
discussions on Tobin Bridge grades last time, it looks like the maximum grade for highways is 4% (maybe 5%).
(FYI, for rapid transit grades: Most info I can find online for railways say 4%, NYC's heavy rail system has the steepest grade being 5.5%, and Riverside said LRT can handle 7%.)
This means if the Chelsea St bridge is rebuilt to support both cars and rapid transit, with a vertical clearance of 170 ft, the lead needs to be 4250 ft for 4% grade - almost getting to Bellingham Square station:
View attachment 46314
(A Tobin-equivalent clearance of 135' would mean 3375' lead, which starts from between Broadway and Box District station. An LRT-only bridge with 7% grade and 170' clearance needs 2428', starting a bit east of Box District near the MWRA building. 7% and 135' needs 1928', which starts at Marlborough St and avoids most of the neighborhoods.)
While I guess a good thing is that the northeast side of this ROW is nothing to worry about, looks like this will still have significant impacts to residents in east Chelsea around the Box District station's area. However, this is turning out a bit better than I thought. And it will probably be much more manageable with a cable car.
So this seems to disagree with
@TheRatmeister's opinion that anything other than a tunnel is ruled out: it's technically feasible, the question lies more in politics. However, I think
a new bridge will likely have to be for transit (and bike trail) only, as the rail ROW doesn't have the width to support both transit and cars. This points to keeping Chelsea St bridge untouched for cars, and building an adjacent bridge dedicated to transit.
(On the other hand, I do think any above-water crossing not using this ROW is out of question due to neighborhood impacts, even though they would be closer to density.)
Miscellaneous
This is actually an interesting connection that I've been thinking about lately.
- There's a great argument for treating Back Bay as part of downtown. In that case, northside Red Line probably has the worst possible connection to Back Bay systemwide.
- Hot take: This is also the best way to improve trips served by the T1 bus (from a demand-only standpoint). Not a route along Mass Ave, and not a route further west. My anecdotal observation is that many T1 riders seem to be going to Back Bay (and not, or at least in addition to, transferring to the Green Line further west), so a more direct connection to the heart of Back Bay area, rather than on the periphery like Hynes station, would really help.
- And of course, an additional connection to regional rail and Amtrak.
I had been crayoning a rapid transit route like this - you may call this a
"mini ring":
View attachment 46315
This may not look like an obvious "Urban Ring" at first, but it's really a hybrid between a circumferential route and a "radial" route centered at Back Bay. Short and sweet. (You can extend it further into Charlestown and South Boston, of course.)
The obvious problem is... $$$$$$. Not only does most of the line need to be deep bored
(C&C under Third St, Dartmouth St, E Berkeley St etc may all be technically feasible, but between utility relocation, landfill, and political challenges due to impacts to residents, I doubt it), but it involves a new Charles River crossing
(that has to be a tunnel unlike BU Bridge), abutting tall and/or historic buildings in Back Bay... etc.
But yeah, back to your point, a cable car between Kendall and Back Bay may be a very intriguing way of serving the main purpose of this line while potentially having a much lower cost.
(Edit: On second thought, I'd combine the Copley and Back Bay stations into one. The platform can be placed on Dartmouth St between St James Ave and Stuart St, which avoids the Pike and most historical buildings in this area (Boston Public Library, Old South Church, Trinity Church). Short pedestrian tunnels connect the Pink Line station to OL Back Bay (370') and GL Copley (310' for GL outbound). This reduces cost and travel time, although may increase traffic at the station. It also offers an in-station transfer between GL and OL/GLR, which is valuable in a GL Reconfiguration world.)