The Sept. 2010 Amtrak NEC vision statement doc just outlines the possible routings. Nothing's agreed upon, and the only one that was ever studied was an old study of re-use of the Air Line with revised routing through Willimantic. These are the ones as I understand them, with engineering/ops + ridership pros & cons:
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West leg
-- **PREFERRED ROUTING** - MNRR Harlem Line to Brewster (speed upgrade, switch to overhead instead of 3rd rail) --> Beacon Line to Newton (refurbish, restoration of 2nd track from historic inland route) --> new ROW to Waterbury (old route partially obliterated by I-84 construction in the 50's...some of it is trailed, some of it outright gone and probably rebuilt adjacent to 84) --> Highland Line to New Britain (restoration of full 2nd track from historic inland route, likely tunneling for 1/2 mile through downtown Bristol to eliminate the hairpin curves for a straight routing...very likely to be full 80 MPH CDOT commuter rail by this point) --> New Britain Secondary to Newington (likely to be restored for CDOT, even if the idiotic and hopefully killed busway has to get ripped out for it) --> Springfield Line to Hartford.
Pros: This was the historic cross-state ROW and has the ROW capacity and (relative) straightness to work. Not a burdensome number of grade crossings. Harlem Line junctions in the right place and can be tri-tracked without too much difficulty to handle the traffic. MNRR can handle swap of 3rd rail to overhead on existing fleet. Max ridership levels and new Amtrak riders. Max congestion mitigation by following I-84. Will get heavy commuter rail use the whole length.
Cons: Newtown-Waterbury gap a hugely expensive fill, likely lots of citizen opposition...this is the high-potential dealbreaker. Downtown Bristol tunnel necessary...expensive, citizen opposition likely. Harlem Line would have to switch to overhead wire from 3rd rail to get HSR-like speeds...very expensive.
-- **NO-BUILD ALTERNATIVE** - NH Line --> Waterbury Branch --> Highland --> NB Secondary --> Hartford
Pros: Much cheaper. Existing infrastructure alternative if Newtown-Waterbury a no-go; short downtown Bristol straightening the only excursion off existing ROW's. Skips a lot of shoreline congestion.
Cons: Still has to deal with New Haven line. Waterbury Branch has more limited expansion capacity and slower speeds than the west-east routing, may require straightening or relocation around Route 8 to get speeds up (more expensive). Still have to straighten out downtown Bristol. High ridership, but not as much relief as linking Waterbury, Danbury, and the Harlem Line because laid on top of all-existing service to Waterbury.
-- Dual route: as above via NH Line, with traffic split on new routing via JFK Airport, various LIRR lines, and tunnel under Long Island Sound.
Pros: Splits congestion onto existing CR lines. Brings Amtrak to largest airport in Northeast.
Cons: Long Island Crossing a non-starter...was proposed time and again on this routing for highways, and costs never washed. Congestion still significant because of NH Line and LIRR use. LIRR would have to be equipped with overhead, which is more difficult than on MNRR.
-- LIRR -- > Long Island Sound crossing to New Haven --> Springfield Line
Pros: Springfield Line existing, straighter, has expansion room to handle traffic and near-HSR speeds. Avoids entire NH Line. Slightly better Sound crossing route than Bridgeport. Max total ridership potential of all routings (not as much all-new Amtrak riders as west-east route). Links MNRR/CDOT and LIRR for thru service.
Cons: Sound crossing a non-starter unless paired with revival of the oft-proposed I-91 to I-495 interstate crossing...which itself has been ruled a non-starter each time. LIRR congestion and speeds not significantly better than NH Line. Switch to overhead much more problematic for 3rd-rail exclusive LIRR...may have to retain both modes at great expense.
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East Leg
-- **STUDIED ROUTING** - Hartford --> Vernon on historical inland route --> Re-routed straight HSR ROW bypassing twisty Bolton Notch portion on state land acquired for long-proposed I-384 Willimantic extension (maybe along highway if it gets yet another revival) --> Willimantic --> restored Air Line to Blackstone, MA --> small re-route on P&W mainline to Woonsocket, RI --> restored B&P trestle back to MA --> MBTA Franklin Line
Pros: Recycles historic Air Line high-speed ROW. Utilizes mostly low-traffic active or landbanked ROW's. Re-route on I-384 land easier than most other all-new ROW's because state already has had multiple EIS's performed every decade when the highway gets revived. Could potentially be built simultaneous with highway.
Cons: Misses all population centers between Hartford and Boston...low commuter rail utilization and probably none whatsoever Willimantic-Woonsocket. New ROW still a community opposition problem because of all the highway spats. Franklin Line can't be expanded beyond 2-track, and still has to merge with NEC for an unexpandable congested stretch in Boston.
-- **NO-BUILD ALTERNATIVE** - Springfield Line --> B&A to Worcester & Boston
Pros: 100% existing Amtrak, already used for inland Regionals. No ROW acquisition, B&A has almost no grade crossings, and last 4 will be long gone by then. Easily expandable ROW's (Springfield Line can go 3-track, B&A 2-track to Worcester, 3-track inside of Worcester on existing ROW's) to handle congestion. Affordable...speed upgrades and electrification only. Springfield line likely to get HSR speed upgrade before inland NEC built. B&A a future HSR upgrade in its own right as Albany/Empire Corridor connection. Can be done no-build if all other routings get shot down, including West routing. Heavy commuter rail usage NYC-New Haven, New Haven-Springfield, Worcester-Boston. No intermixing in Boston with NEC. Hits max number of large cities. Can/will continue to be used as roundabout lower-speed routing anyway even with a more direct inland route built.
Cons: Out-of-way routing means no time savings from NEC. No relief whatsoever from NH Line congestion if West leg gets killed too. Very heavy freight traffic may limit speeds Springfield-Worcester where ROW is max 2 track. B&A very twisty Springfield-Worcester...curve eliminations and bypasses not easy to do because of hills, unlikely to top 115 MPH west of Worcester except for short stretches.
-- Hartford --> Vernon on historic west-east route --> new ROW on I-84 median to Sturbridge --> Charlton Depot on MA-owned conservation land for long-canceled I-84 extension --> B&A to Worcester-Boston
Pros: I-84 Highest-speed ROW segment in entire northeast...150+ MPH sustained possible. Joins 2 largest east-leg metro areas with probable 30-minute travel times. No land acquisition...active ROW's + extremely over-wide I-84 + state land in MA. No structural relocation of bridges required. Joins B&A at spot where it straightens out for higher speed and has full room for 3-track to Boston to bypass freight and MBTA congestion. Easy environmental permiting on I-84 since carriageway was 100% reconstructed/regraded in 1980's. No community opposition on I-84, very sparse population Sturbridge-Charlton. Hartford-Worcester commuter rail potential. Huge I-84/Mass Pike traffic relief. Highest-revenue new freight potential, linking NECR to Worcester double-stack interchange with all 3 regional Class I's & II's.
Cons: New ROW considerably more expensive than existing. 1 3/4-mile long tunnel bore required in Tolland where I-84 on too-steep a grade for acceptable speeds; 1 potential shorter tunnel also in Tolland. Some EIS risk on MA conservation land (I-84 extension never got far enough in 60's to tackle wetlands).
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Of these the officially-preferred pure west-east NYC routing is the only one that totally bypasses NEC congestion and can do near (but probably not true) HSR speeds while tying together a lot of dense population and getting max commuter rail utilization. The Sound crossings are utter nonstarters, and any of the no/minimal-build options on the NH Line are half solutions. This one's also by far the most expensive and fraught-with-peril option on community opposition. If it ever gets built I would expect it to be the utter last phase of Amtrak's whole 30-year NEC corridor vision.
Of the east leg ones the I-84 route has the max capacity and speed upside and would be biggest bang for bringing a significant stretch of honest-to-God "true" HSR to the NEC. Attractive because of the land ownership, untapped B&A capacity, big freight upside, and linking Hartford, Worcester, and Boston in proximity they've never had before. Also outright encourages growth on the Springfield line and B&A-to-Empire Corridor to fill out the radial NEC network. But if the U.S. just doesn't commit enough to considering "true" new-ROW HSR akin to how it did with the Interstates, then even a pretty reasonable-risk one like this is going to be a tough sell. Of other routes Air Line has decent speeds and full ROW availability but misses Worcester and nearly all population between Hartford-Boston, doesn't get the commuter rail utilization to really offset the electrification and track maint costs, has less expansion capacity, and doesn't cut travel times so dramatically that Hartford-Boston get dramatically closer-aligned. Capital costs nice, but operating costs not as much. The Springfield/no-build alternative already exists as low-speed, will probably exist as high-speed in its own right independent of anything else, can serve as the interim route for both east and west legs until they're connected. That's the fallback if nothing else can get done. Population linkage and utilization would be very good...so good it would be its own tertiary route even with a new bypass. But travel times would be no better at all because of the out-of-way mileage involved. At least it's likely to happen regardless of iffy U.S. commitment.
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Now, before johnwayne jumps down our throats again with more condescension...remember that this is just a vision statement. They're stating a future goal on the public record and establishing the shortlist of routings to poke around with gingerly for future study. It's not a financial commitment. Initial studies, if they happen, are a trivial expense. None of it has to happen. None of it will happen if the country isn't willing. But first step to doing anything ever is to state your objective in plain English. And that's what this is. It's not fantasy daydreaming. It's stating an objective and the what's/where's/why's of it. Nothing more, nothing less.