If you back up and say the problem is *mobility* (not "air travel) and frame it for the Boston Metro area (not "Boston") you'll see that added capacity at Logan is not necessarily the answer--and certainly not the cheapest.
It may be considerably cheaper to improve Acela services to hourly or even twice-hourly. This would cause the airlines to cut back service and free up capacity (now devoted to BOS-NYC) for all those other things (like transoceanic flights) that *only* Logan can do.
This is what has already happened on LON-PAR routes (which used to be Europe's busiest air corridor). As the Eurostar train came to dominate, the airlines suddenly found they had slots at LHR and CDG that they could allocate to long-haul flying. The same will happen in Boston. The Tories estimated that 20% of London's flying was to markets that high-speed rail could serve better...not quite enough to save a whole runway (they're considering a 3rd for LHR or a second for LGW) but darn close.
Airport guys talk about how much new capacity we need without ever asking if another mode could better accommodate the traffic.
Trains have a proven ability to "drain" trips off the bottom of an airport, thereby effectively adding to capacity for long-hauls.
Reliever airports have been less successful, but PVD and MHT have the potential in the long run to get flights to hubs like FRA and CDG. Yes, BDL's flight to AMS didn't work, but if demand is really growing as we think some of what is thought of as "BOS" demand would actually be happy to fly from PVD/MHT/BDL in future years.
It's not going to be considerably cheaper to do anything with the Acela because of the political hostage-takers otherwise known as the Connecticut Marines Association - there's no way to get to T.F. Green (which isn't even adequately prepped to see Amtrak service of any kind) without being stuck on the Shore Line down through several bridges which we just can't seem to find the will to keep closed any more often than they currently are. 39 trains a day - absolute limit. The Marine Trades Association steadfastly opposes any more.
On the north side, Acela service to Manchester is laughably pie-in-the-sky with no electrification, no rail link, and no active lines out that way to even start from. Not to mention that readying the Lowell(-Concord) Line for high-speed service is going to be far, far more involved than simply extending commuter rail service into New Hampshire - eliminating grade crossings, reconstructing several stations, building several new ones, double-tracking (triple-tracking??) 100% of the line and probably smoothing out a few unfortunate curves. Mission creep to the extreme versus a relatively straightforward commuter operation with the potential to have a Regional-esque service stapled on after the fact. I'm not saying all of that shouldn't be done eventually anyway - but it's a 2050 discussion, not a 2015 one.
Fortunately, the solution doesn't lie in the Acela anyway. Commuter Rail service to KMHT and expanded service to KPVD is sufficient if limited-stop expresses can be introduced to keep the travel times down to between 45 minutes and an hour. Amtrak Regionals today do BOS - PVD in about 40 minutes and less extreme padding would let them do PVD - BOS in about 40 minutes as well. We'll call it 50 even for the projected time on a roundtrip to the Airport. In the other direction, 50 minutes isn't possible without the same sort of exorbitant expenditures needed for Acela service, but that's okay, because 75 minutes is probably doable on day 1 with the introduction of express service that skips every stop between KMHT and North Station except for Nashua (optional), Lowell, and Anderson RTC.
The reason why KBDL didn't work, won't work now and never will work is because driving times from Boston to Windsor Locks clock in at an even 2 hours. Meanwhile, Manchester is 1 even and Warwick is 1 and change. Reasonably heavy traffic, such as you might encounter during rush hour, inflates those times by 15 minutes or so. Still, my point is clear - roughly an hour, give or take 15 minutes based on modal choice and externalities like traffic, puts Manchester and Warwick both well within range of Boston (hell, 45 minutes to an hour is probably about how long it takes you to get downtown from Brighton or Braintree on an average day). With extremely limited investment needed on the KPVD side, and no investment that wasn't already going to be undertaken as part of New Hampshire's Capital Corridor anyway on the north side. Very simple, very easy.
The best case scenario you could ever possibly hope for at KBDL is a two-hour drive, or an hour train trip into Hartford plus a transfer to a different train to Windsor Locks followed by a shuttle to the airport, likely ending up taking you longer than the two-hour drive. Nobody in the Boston metro market is ever going to be happily flying out of KBDL, but plenty of people in the Boston metro market will happily fly out of KPVD or KMHT.
