I've never understood why it is such an important thing. If in fact the South Station expansion is going to solve much of the foreseeable capacity issues that NSRL was to have solved (at a fraction of the cost) then NSRL should only be pursued if the other benefits are worth the added $5B or so in capital costs.
I think it's right to have a healthy skepticism of the NSRL. That's why I think its cost will only be justified if it results in a major transformation to the region's transportation.
Now, SSX will partially solve one problem, which is the capacity limitation at SS. That will enable increased levels of service on the south side lines and Amtrak. That will enable doing more of the same, plus DMU service, as planned. But while the DMU service will be an improvement, it's still probably going to be 30 minute frequencies at best on most lines. Maybe inconsistent 15-20 minute frequencies on the Fairmount line, it's been speculated, but that's it. That's not a revolutionary difference, it's an incremental improvement. And that's fine for what it is. It won't do anything for the north side, and it probably won't draw that many new riders.
Facilitate suburb to suburb travel - Why on earth would we want to do such a thing? Wouldn't suburban depopulation be a better goal, at least until we've densified all of the underutilized areas surrounding our rapid transit stations (Sullivan, Maverick, Andrew, Forest Hills, etc, etc)
I totally agree with you that, in the near future, we need to grow the neighborhoods around those squares (Sullivan is particularly egregious in its current form). There's much existing transit investment which is not being utilized. However, two things: eventually the cost/benefit of redeveloping near existing stations rises too much, and, eventually the cost/benefit of increasing the capacity of the rapid transit lines also gets too high. Not to mention, there's seemingly intractable bottlenecks at core transfer stations such as Park or DTX which will only get worse over time.
At some point we need to address these issues. Transit expansion is one option. We could build all-new lines to relieve crowding on existing lines and open up development options in different places. That's a very difficult proposition however, because this is Boston, and most places are built up enough to resist change and the kind of disruption that the construction of a new line would bring.
But we already have many rail corridors, dating back to the 1830s, running through the city. And they even have existing passenger service. So, another option is to increase that service until it approaches rapid transit-like service. This is not a new idea, it's quite common in history. The Dorchester branch of the Red Line is one example. The Revere extension of the Blue Line is another. The "D" branch of the Green Line. The SW corridor and the GLX are somewhat similar, except those corridors are big enough to support both rapid transit and traditional American commuter rail.
So, when we say suburb-to-suburb travel, we mostly mean the inner-suburban neighborhoods which in many cases have been incorporated into the city of Boston. Or the surrounding urban, or semi-suburban towns and cities.
For this kind of increase in service to work, to become rapid transit-like, and to be utilized as such, it needs both frequency and connections. That's where the NSRL comes in. SSX can buy you frequency to an extent. But not as much as thru-running. And SSX doesn't solve the problem that all the increased service terminates at SS. We can already see how the Silver Line-Waterfront is dampened by its lack of connections, an issue that was supposed to be addressed by SL Phase III, but that plan is too costly. The same problem afflicts Fairmount-Indigo. And it will afflict any service which just terminates at South or North station.
Certainly, you can hop on the subway at those stations. But that puts us right back at the burgeoning capacity problems on the core subway and the core transfer stations.
The NSRL may also be the only feasible option for major transit expansion that passes through the heart of Boston. So if we are going to focus development around transit stations, and enhance the MBTA, this may be our best shot at doing so around corridors with lots of room to grow. I suspect that, other than the NSRL (and Red/Blue), most transit expansion will be surface-based.
Better distribution for commuter rail customers - If this becomes the main argument in favor, it gets pretty hard to defend. If the current "bad" situation is getting off at North Station and having to take a 12 minute rapid transit ride at 4 minute headways to get to Back Bay, or walk 15 minutes to the Financial District, what will the delta be for the "good" situation afforded by NSRL? And how many of the trips to be served by NSRL have already been siphoned off by Hubway?
The trips I am thinking about are more like: connecting across the platform to get to places like Ruggles, Yawkey, Boston Landing, Newmarket, Porter, UMass, Lynn, Malden, Readville, etc. Either places that aren't covered well by the existing subway network, or places for which the number of transfers is excessive, or involves adding too much crowding to the already-busy transfer stations.
