Hypothetical T map (from Reddit)

The real potential benefits of the NSRL are the fast one-seat and two-seat rides between close-in points, and the access to more lines from both terminals. Through-running Amtrak service would be convenient, but that's a small percentage of the potential capacity.

Is anyone going from Greenbush to Fitchburg, or Rockport to Worcester? Unlikely. But all the people working by Porter and Malden and UMass and Braintree and Ruggles and Yawkey, et cetera, are often coming from lines that don't serve those stops. So they have to ride the subway anyway, clogging up the downtown transfer stations. But set up the Link - and separate your 495 expresses from your high-frequency 128 locals - and you can use those locals as express subway services. Get on an express at Worcester, wait less than 20 minutes (average just 10) at South Station for a Waltham local, and you can get to Porter or Waltham in minutes. Or I can get on a local at Yawkey, change at North Station to a Reading local, and meet my sister in Melrose. If the Subway Line That Shall Not Be Named turns out to be impossible to build effectively - and it may well be - then fast through-running commuter services may be the only effective may to take the load off the downtown transfer stations.

When talking about the potential NSRL, don't underestimate the pull (at least politically) that through-running Amtrak service could have, especially when it comes to funding the darn thing. Just think of the political capital (not to mention actual ridership) that could be gained from, say, running the Downeaster direct from Portland (or Rockland!) to NYC and beyond, not to mention the doing the same to Nashua, Manchester, or Concord. While it certainly would amount to <10% of the NSRL's traffic, getting NH and Maine on board could really help to open the federal spigots.

Back to MBTA-related topics though, the idea that really excites me is that of turning as many of the CR lines, at least inside 128, into RER-style rapid commuter service. Basically Paris, like Boston, is a dense but sprawling city with a number of little epicenters of growth clustered around the old city core, and like Boston, with the Charles and the Harbor, Paris has to contend with a few geographic constraints on where that growth can actually happen. Furthermore, Paris, like Boston has a fairly extensive but antiquated rapid transit system that struggles to handle peak ridership and unfortunately cuts through a dense historic downtown where the idea of building whole new rapid transit lines from scratch would be all but unthinkable due to the cost. That issue of cost, along with the difficult geographic constraints, was also the thing that deep-sixed the feasibility of building any sort of connecting ring line in Paris.

So what did the RATP do? They used their existing commuter and intercity trackage and built the RER lines along those right of ways, with minimal new tunneling as needed to connect the gaps between the lines. The RER, being based on commuter lines, funnels multiple feeder lines into a handful of central tunnels. Since it's based on radial commuter lines, it's unable to offer any sort of "spoke-to-spoke" connectivity, but it makes up for that deficiency with almost rapid transit-level headways, and since it's electrified it can use fast-accelerating EMU's that hit 60+mph speeds, reducing the time from one point on the edge of Paris to another to a matter of minutes, all without forcing commuters and travelers to use the slower metro lines or relying on expensive new-build ring lines.

With all of this in mind, my hunch is that the closest thing to a "perfect" solution to the T's access, capacity, and connectivity woes for a "reasonable" price would be to try and emulate the RER system as best as they can. RER-ify the Fitchburg and Rockport lines inside Waltham and Lynn to the North, and do the same to the Fairmount line (terminating at 128/Westwood?) and the Worcester/Riverside line to the South. All you'd need is the NSRL to connect it all, and you'd take care of a great deal of the inside-128 traffic that the line which shall remain unnamed would serve. And you could probably do it for barely the cost of the GLX and SCR combined.

Has anyone else here ever experienced the RER firsthand, and do you think this idea might work here?

When in Paris, I didn't use the RER *much* but, when we did, it was a godsend. When we went to Versailles, it was ONE metro to ONE RER ride. Same thing for going to the airport. A friend of mine was working in Boston and the idea of Commuter rail to subway to subway to office was horrible. If the NSRL existed, it would have been Commuter Rail to subway as he was coming from NH but the office was in Dorchester (south of the Boston CBD).

If the T were to RER-ify the lines inside 128, I think a different style of car would be needed for anything beyond the current core that we have. The new cars would have to be more like the Bluebirds, M8s, or RDCs, in that they would have vestibules, lateral seating as opposed to longitudinal, and could possibly (though not necessarily) operate on the same tracks as regular size passenger and freight trains. This would be especially useful in the areas on the map where Rapid Transit parallels/displaces Commuter Rail service. In addition, regular subway style cars like the ones currently in service would make short turns within the core system, such as Harvard-Columbia, Sullivan-Arborway, Chelsea-Harvard, and Airport-Watertown.

The light rail lines are a whole 'nother story.