I'm going to say that NSSP and NSRL are *not* competing. On several walksheds that NSSP (the city's name for North Station Sea Port) directly serves demand that NSRL cannot, partly because NSRL ends up being farther away from places simply because it will be so deep under that it imposes a lot of vertical commute time (particularly if they delete Central Station)
Weak Case (where it
seems to work like NSRL, but because it depends on where you're going, ends up being complementary)
1)
North Station and Haymarket and Government Center Access from the South. NSSP offers a better option than exist today, and for some might remain preferred. For
Government Center, NSRL would still require an indirect route from South Station or backtracking from North Station. The new
State Street HQ (and any other developments in Bullfinch triangle) probably has a lot of commuters who rely on a CR/SS commute who will want a way to get to Congress@Haymarket via a connection at South Station. This will be good for them, even though, if it has the right vertical circulation, a NSRL could be better (and no connection)
2)
FiDi And Leather District from the North. Here again NSSP is better than today, not as awesome as NSRL would be, and for some people the bus would be better even if NSRL existed, because NSRL platforms may be farther/deeper than a surface line on Congress St
Strong Case (where it CLEARLY does things that NSRL wont)
3)
Boston Wharf Co, Convention Center, Drydock Ave (the 3 stops on the
Summer St Corridor). OK, not technically in the scope of NSSP, but it is exactly where Northside commuters arriving at NS and Haymarket, and Westside Express buses at Post Office Sq need easy connectivity, and that connectivity starts at the curb on Congress St. And, frankly even under NSRL, South Station needs a way to get to the Convention Center, which becomes easier to provide if it is attached to NSSP. (also a great alternative to get to
Seaport Parcels N P L5 L6 and vicinity, given a stop at Summer @ West Service Rd
4)
NSBL: North Seaport BUS Link. This is where NSSP starts to look like NSBL). Picture the buses that today terminate at Haymarket or Post Office Sq being extended to the Seaport (faves: 111, 92, 93) but really anyplace on the North that there are lots of bus riders who live far from a CR stop.
5)
Airport Service. The Silver Line should have some routes that run North Station - Congress St - Airport. Can't do that with NSRL.