Since transit to Logan has come up in other threads lately, here's (I think) the best possible solution. Assuming a world with NSRL, Logan should be connected to the commuter rail. And it should look like this:
View attachment 56420
That blue line is a TBM'd tunnel, a little longer than three miles, that would connect directly to the NSRL tunnels (without a connection to surface North Station). The airport station would be directly under central parking.
Here's my thinking:
People here have pointed out that the airport largely serves two different groups of people. The first is locals using the airport, which skews heavily suburban. The suburban group is a big sign that a rail connection most naturally fits on the commuter rail network, not rapid transit. I don't think a rapid transit connection is a bad idea, but it's not the best fit. This proposal gives lots of people in southern suburbs and even Rhode Island a one-seat-ride directly to the airport. For those with a two-seat-ride, the transfers will be really easy. For many it would be a cross platform transfer, and for others it would be a simple up and over without having to leave the underground NSRL stations.
The other group of travelers is visitors to Boston, who are mainly headed downtown. This might be
the best Airport-to-Downtown rail link of any city in the world. This takes people directly to major destinations and hotel clusters at North Station, Aquarium, South Station, and Back Bay. And it would hit each stop extremely fast. The curve I drew has a half-mile radius. I can't figure out exactly what is required, but that'd roughly let the trains run 80mph, I think. That would be Logan to North Station in 3-4 minutes. Airport to South Station in maybe 8. For any downtown station, this would be faster than driving, even if there is little car traffic. Under common conditions, the train could get to Back Bay before a car could get from Terminal B to the Ted Williams Tunnel.
Also, for people living in the city trying to get to the airport, this connects to every rapid transit line.
Is this a waste to have such a short commuter rail line? No, I don't think so, for two reasons. First, Logan is a major trip generator. There are 17,000 employees who could benefit from this.
TSA regularly screens more than 70,000 passengers a day, and I'll assume the inbound is about symmetrical, so 140,000 people coming and going. If this line captures just 15% of those people, that'd be in the range of the busiest commuter line today. Second, branching off the north side of the NSRL tunnel takes advantage of slight issue with NSRL. Namely, there are more branches and trains coming from the south side than the north. Adding this new north side branch gives a new place for trains to go. In a Regional Rail world, the 7 south side branches could easily send a total of 8 trains an hour to the airport, meaning a train every 7.5 minutes.
Cost: expensive. But I don't know, I'd be curious to hear how expensive people think. The commonly proposed transit-to-Logan is extending the Silver Line tunnel through Seaport, then a new cross harbor tunnel. That is also super expensive, and I don't think I've ever seen a version that put a station in so useful a location. For this commuter rail proposal, this is all TBM, so the setup is expensive but then the cost per mile is relatively cheap. Also, in a project like this it's the stations that get expensive, and here there is only one station. Building under the parking garage could get expensive. Demolishing a small part of central parking and putting a station in its place would be much, much cheaper. And the station wouldn't have to be deep. Even if the tunnel is 100ft deep under the Mystic River, there's over a mile straightaway under East Boston to gently bring the track up to surface level at the airport
One far distant benefit is this could also by used by Amtrak. Having the Northeast Regional start/stop there could make Logan the best airport for some people into Connecticut. The East-West Rail project could do the same for people in Springfield, or even as far as Albany. (I lived in the Berkshires for a time and would fly out of Hartford or Albany, but sometimes it was easier to skip a flight connection by just driving to Boston.) In some future with real high speed rail in the US, a station at Logan could be used to eliminate some shorter connecting flights.
One more thing that is maybe wishful thinking, but perhaps something like this gets NSRL to actually happen. Most people simply don't care about NSRL, or transit generally. But people often
do care about transit to the airport. Some argue governments tend to
overvalue connections to the airport. So something like this might get the public and politicians to care about NSRL. We can say "Build NSRL, and then Boston can get the best Airport-to-Downtown train in the world." It might help.