First of all, that's a pretty creative proposal! I've seen a few pitches of light rail lines connecting Seaport to South Boston and beyond, but can't recall anyone suggesting an extension from the South Station end to Broadway.
That said, unfortunately it may face some issues getting out of South Station.
@F-Line to Dudley has mentioned a few times that an extension of the Transitway
south of South Station is likely infeasible due to vertical space constraints, as the current South Station SL level and the bus turnback loop to the south are just above an I-93 ramp.
A few quick searches gave me the following posts:
this,
this and
this.
I'm not sure if a trajectory to the
southeast is feasible by avoiding the conflict with I-93, especially if you plan on abandoning the bus loop and making the tunnel LRT only.
I'll say that's a no, for two reasons:
- The 7 bus has substantial boarding east of L St, which your proposed route does not capture.
- The 7 also has many passengers alighting in the Financial District and Downtown Crossing, which are also not served by your route.
These are best illustrated in the
bus route profile as shown below. Keep in mind this was pre-Covid, when the 7 ran frequent short-turn trips at South Station, resulting in more than half the AM peak inbound trips not serving downtown. This is no longer true today.
View attachment 39267
I took a look at the more recent
data from Fall 2022. On weekday inbound trips in a single day, on average:
- 295 riders board between City Point Terminal and P St. The O St and P St stops, where most of these boardings occur, require a 10-min walk from your route.
- 200 riders board at N St (5-min walk from your route).
- 559 riders board at the L St stops, which are on your route.
So about 47% of riders board at stops that will be removed in your proposal, with half of them requiring a non-trivial walk.
And then there's additional walking at the other end of the route. Comparing South Station alightings and downtown alightings:
- 591 riders alight at South Station.
- 673 riders alight at the two downtown stops (Franklin @ Devonshire, Otis @ Summer). The former accounts for roughly 2/3 of these alightings (457 riders). Franklin @ Devonshire is a 6-min walk from South Station, but some offices may be further north.
Once again, more than half of the existing riders will be inconvenienced by the proposal at the downtown end alone.
An argument can be made that it's manageable and worth the tradeoff for better service, but that's still a very significant amount of riders - probably three quarters of people who take the 7 today will need to walk more to take your proposed line.
Handling SL1/3 in a world with the Transitway being turned into LRT has been discussed somewhat frequently here. Among current ideas, the two most notable ones (that do not involve mixing LRT with BRT in the Transitway) are:
- Send both bus routes to Summer St, where there's some effort at BRT infrastructure being planned, and terminate at street level at South Station (or the new bus terminal under construction, which will be much closer to the station - correct me if I'm wrong)
- Make the SL1 express from South Station to Logan Airport, skipping Seaport; SL3 goes to Summer St
- Rationale: Relatively few riders utilize the Logan-Seaport connection, as most SL1 riders are travelling either Logan-SS or Seaport-SS
- SL3 mayyy be able to terminate within Seaport in a post-Red-Blue world
I do think South Station is a non-negotiable for both routes at least before Red-Blue. (Most of) SL1's riders from Logan are going to South Station to connect to the subway, especially the Red Line, and commuter rail. SL3's riders - from both Chelsea and Blue Line transfers - additionally see South Station as a way to get to the Financial District, with a one-seat ride from Chelsea. A forced transfer at Silver Line Way or WTC just doesn't make the cut, especially for SL1.
SL3 does need to keep serving Seaport, as it has significant Chelsea-Seaport and Airport Station (BL)-Seaport markets.
@Riverside has done some excellent
analyses on ridership patterns on SL1 and SL3. He may be able to add more to the discussion.
IIRC, the NSRL tunnels will be way below the Silver Line platform level at South Station, so that itself is not a concern.