If new high-speed rail construction has to largely follow Interstate corridors, then a new line from New Haven to Boston can serve either Providence on I-95 or Hartford on I-91/84/90, but not both. However, there’s still the possibility of building a completely greenfield alignment between Hartford and Providence; the FRA is investigating this as option 13 of NEC Future and Amtrak is proposing this in the latest update of its Vision. Since the terrain is hillier than on the coast, it requires some investigation as to whether it’s possible to connect Hartford and Providence without excessive tunneling. The answer turns out to be yes, but only at the cost of slowdowns both north and south of Hartford that impose real costs relative to following I-95: construction is likely to be more expensive and travel time including a Hartford stop is 9 minutes longer.
I believe the alignments depicted in this map are near-optimal for New Haven-Providence via I-95 and via Hartford. The New Haven-Hartford alignment is similar to that of Penn Design with two major differences: Penn Design diverges to cut off some curves near Hartford, but to guarantee sufficient curve radius it has to slice a significant chunk of downtown New Britain; and Penn Design also straightens the route in New Haven with a tunnel, which is unnecessary as the time savings do not justify the expense. Amtrak prefers getting to Hartford from Danbury, but to get there from New Rochelle requires long suburban tunnels, which my alignment avoids. I have not seen a detailed Hartford-Providence alignment, and I drew a line based on Google Earth elevation with an eye toward avoiding tunneling, which means there may be some further optimization, for example a rigorous cost-benefit analysis of viaducts versus curve avoidance.
The Hartford-Providence greenfield route has no tunnels except in Providence itself, where the line tunnels under Olneyville for about 2 kilometers. In Connecticut the route has many viaducts, but does not need to tunnel through the inland hills. Rather than giving detailed cost estimates, which are possible but not with sufficient reliability or precision, I am going to qualitatively describe construction challenges for each route and then the differences in travel time, which favor not serving Hartford. The final decision should boil down to the question, what cost is it acceptable to impose on New York-Boston travelers to allow for service to Hartford?
The I-95 route is zero-tunnel. The Hartford route has no tunnels in Connecticut, but requires a tunnel of 1.5-2 km in Providence. There exists an old railroad alignment going around the river and connecting Providence to the west without a tunnel, but the right of way was given away and to restore it would require some urban building demolitions as well as configuring a flying junction under Route 6 while also slowing down trains further.
The I-95 route has significant challenges in river crossings, since it is close to the coast. Three difficult crossings are needed, of the Quinnipiac, the Connecticut, and the Thames. The Connecticut only requires a span parallel to I-95. The Quinnipiac requires a new span parallel to US 1 and I-95 and a new approach from Union Station; there is space for this approach, and the curve radius can be kept to at least 500 meters, but it requires work on active track. The Quinnipiac span can be avoided by using the existing route around the bay, which crosses the river at a much narrower point, but this adds several slow kilometers to the route. Recent construction costs for parallel bridges are $125 million for the four-lane US 1 bridge and $554 million for a signature ten-lane I-95 span; I believe the lower cost is more indicative of the infrastructure required for a two-track rail bridge.
The Thames is the hardest, since the route of I-95 and the terrain make it hard to cross anywhere except downtown New London, a constrained urban location. There is just enough space for a station between the decks, and the alignment may impose further constraints on curve radius. There is more space north of both decks, or alternatively Connecticut could build a third I-95 deck and repurpose one of the existing decks for rail.
The Hartford route has one significant water challenge: crossing the Connecticut in downtown Hartford. There is an existing bridge, but it is single-track and would require a completely new span to be used by high-speed rail. It is also used by freight, but only by a short branch line that could be bought out.
The Hartford route also needs to cross the Scituate Reservoir, adding about 3 kilometers of viaduct. However, there is a choice of where exactly to cross it and not much development on its banks, making construction easier than on I-95 or across the Connecticut in Hartford.
Terrain and viaducts
I-95 is substantially flatter than the inland route. Only two short segments require significant overland viaducts and earthworks: the transition in southern Rhode Island from I-95 to the Shore Line, and the curve west of New London cutting off curvier parts of the Interstate. The transition is in total 16 km long but only about the western 10 km of it are difficult (of which about half require viaducts and half can be done cut-and-fill), and west of New London there are 6 difficult km requiring a viaduct north of the Niantic.
In contrast, the inland route needs to be on viaduct for a significant portion of the Hartford-Providence section. Of particular note is the Quinebaug River valley, about 13 kilometers of route of which most requires extensive grading and viaducts, as well as some takings in the built-up areas of the towns of Brooklyn and Killingly. The Willimantic River-Mansfield Hollow Lake-Natchaug River complex adds another 16 kilometers, some hard and some less hard; the Willimantic itself is in a deep valley requiring a tall viaduct of about 3 km, and the total viaduct length required appears to be about 8 km. The following 12 km, on the crest heading to I-84, require some earthworks, but probably no significant viaducts.
Urban construction challenges
I-95 has an existing route into Providence. Some curve modifications from East Greenwich northward are helpful for keeping speeds up, but the grade-separated route already exists. The main challenge is fitting regional trains if Rhode Island desires to run them: the right-of-way has room for four tracks but only if track centers are narrowed so much as to preclude tilting, reducing cant deficiency to about 125 mm. At the New Haven end, the main challenge is crossing the Quinnipiac, but once the tracks are east of the harbor, suburban development intensity drops rapidly, requiring only occasional grade separations with roads crossing I-95. Conversely, if intercity trains are all routed through Hartford then no new construction is required for any Rhode Island regional rail.
The major problem then is New London. The entire complex of crossing the city and the Thames is the biggest difficulty in the route, as outlined above in the water crossing section. In addition to geometric difficulties, there are also noise abatement issues, since the track geometry still allows very high speeds (the curve drawn above just west of New London looks like it can be eased to about 3 km, allowing 310 km/h). This is what favors putting the tracks between the two I-95 bridge decks instead of to the north.
The inland route has far greater difficulties. First, it needs to carve a partially new route into Providence, hence the Olneyville tunnel; however, it also leaves the Providence built-up area much faster, within about 6 kilometers vs. 24 for the Shore Line. In New Haven and Hartford it can for the most part transition between legacy rail routes and expressway corridors, but a substantial portion of the route is in the suburbs of those two cities, which requires more grade separations and makes curve modification harder. There are also noise abatement issues, though Shinkansen trains skip some urban stations at 300 km/h, so those issues are more about cost than about speed limits.
There are several alignment choices north of New Haven. The one I used on the map follows the Providence and Worcester’s Middletown Branch right-of-way and thence I-91, but it is equally feasible to take a more westerly route via the Amtrak line transitioning either to I-91 or Route 15; both options involve grade crossings and extensive suburban construction. In all cases, the trains are almost continuously in built-up area from New Haven until 19 kilometers east of Hartford. Grade separations have the full cost of urban or dense-suburban construction, and moreover, the transition to I-384 east of Hartford requires some additional takings.
Total new construction
This is the primary advantage of I-95, cost-wise: the track already exists from Kingston north and requires only minor facelifts. The New Haven-Kingston construction is just 124 km, whereas between the splits with the legacy Northeast Corridor in New Haven and Providence the Hartford route is 167 km.
With this in mind, nearly the entire I-95/Shore Line segment between East Greenwich and East Haven can be eased to a curve radius of 4 km. New London, where noise abatement prevents running at full speed anyway, can accommodate slightly lower radius, about 3 km on the western approach. At the New Haven end, the transition to the Quinnipiac bridge right next to the station has radius 500 meters, but the speed restriction is minor since it is so close to the station.
Hartford-Providence can also be eased to quite high curve radius. In Rhode Island, once out of the Providence built-up area, the tracks can maintain a 4-km standard, and until the transition to I-384, the worst radius is 3.1 km around Mansfield. However, from I-384 west, things become far worse: the transition to the east has a radius of 1.2 km and seems impossible to increase further, the transition to the west has a radius of at most about 1 km, and the curve west of the Connecticut bridge is 500 m and is slightly farther away from Hartford than the Quinnipiac bridge curve is from New Haven.
It is south of Hartford that things deteriorate. The worst curves on the legacy lines are in Meriden and can be bypassed, but there is a 1.3-km curve in New Britain, on an S with a 2.3-km curve just south in Kensington that makes it unfixable. At the New Haven end there’s a curve on the legacy line, bypassed on I-95 by the Quinnipiac bridge, with radius about 450 m about 2.5 km out of the station.
Overall travel time
The explicit assumptions on trains are aggressive, based as always on the need to keep speeds up in big cities and on the only partially fixable New York-New Haven segment. Trains accelerate like the N700-I (26.74 kW/t, more than any high-speed train that currently exists except the Talgo AVRIL), cant deficiency is 175 mm as on the E5/E6 and on the AVRIL, cant is 200 mm as on the Tokaido Shinkansen, and initial acceleration is 0.89 m/s^2 as on the N700-I. With these performance specs, the minimum curve radius required for a full speed of 360 km/h is 4 kilometers; the Tohoku Shinkansen has such radius and JR East intended to run trains on it at 360 km/h before deciding to reduce speeds to 320 for reasons that are not track geometry.
For simplicity of computation I’m going to ignore grades. Since the I-95 route is flat, with very few grades higher than about 1%, this is justifiable there; it’s a little less justifiable through Hartford because a few segments have 3% grades, but they are also quite limited.
Without any schedule padding, we can set the following speed zones for I-95, measuring from 0 km point in Providence and going southbound:
0-0.6 km: 90 km/h (curve around Providence Station)
0.6-4.5 km: 120 km/h (two 450-m curves)
4.5-7.5 km: 180 km/h (Mashapaug Pond curve is too close to 120 km/h to matter, curve into Cranston is about 1 km)
7.5-17 km: 250 km/h (no curves, trains can achieve 270 in between curves but this would only save 5 seconds)
17-22 km: 220 km/h (curves have radius about 1.4 km and the controlling curve at km-point 17 can be eased a bit)
22-92 km: 360 km/h (full speed to New London)
92-103 km: 310 km/h (speed restriction in New London and the curve north of the Niantic River)
103-162 km: 360 km/h (full speed to East Haven)
162-167 km: 250 km/h (curve around an East Haven hill, though trains can barely accelerate fast enough for it to matter going eastbound)
167-168 km: 100 km/h (New Haven approach)
The time taken to transition between speed zones is the average of acceleration and deceleration time penalty. This gives a technical travel time of 33:40 for nonstop trains. If trains have a top speed of 300 km/h, this raises the technical travel time to 37:28.
Now, let us set speed zones for the Hartford route:
0-0.6 km: 90 km/h (curve around Providence Station)
0.6-4.5 km: 180 km/h (curve north of Hartford)
4.5-6.5 km: 200 km/h (curve into Johnston)
6.5-10 km: 240 km/h (curve west of I-295)
10-57 km: 360 km/h (full speed to the Hampton-Mansfield area)
57-86 km: 310 km/h (Hampton and Mansfield impose a 310 km/h restriction to km-point 67, and trains going eastbound can’t accelerate to 360 before they have to slow down again anyway)
86-88 km: 220 km/h (gentler curve in the transition to I-384)
88-101 km: 200 km/h (transition curve to I-384, further curves on I-384 making speedup between transition curves pointless)
101-103 km: 160 km/h (transition curve)
103-109 km: 200 km/h (minor opportunity to make up time, saves 20 seconds over 160 km/h)
109-110 km: 130 km/h (curve on eastern approach to bridge)
110-112 km: 110 km/h (curve on western approach)
Hartford Station: all trains stop since curves limit time savings from not stopping, as at New Haven and Providence
112-127 km: 250 km/h (New Britain curve, speed increase to 270 km/h in between is possible but saves only about 8 seconds)
127-153.5 km: 270 km/h (Kensington and Berlin curves preclude higher speed)
153.5-155 km: 210 km/h (S-curve precludes easy straightening, and significant speed boost requires significant residential takings)
155-169 km: 250 km/h (this requires straightening the kink around and north of the I-91 underpass, otherwise 210 km/h to km-point 162, 160 km/h to km-point 164, and 200 km/h farther south)
169-172 km: 120 km/h (New Haven approach, legacy line curve)
The travel time is 25:30 for nonstop trains from Providence to Hartford and 16:10 from Hartford to New Haven. With a minute of dwell time at Hartford, this is exactly 9 minutes longer than I-95.
Compatibility with other plans
Although I-95 requires less construction overall than Hartford and the construction difficulties are about comparable, Hartford is more compatible with other intercity rail plans for New England, which reduces the advantage of I-95. Under an I-95 option, it is still useful to serve Hartford (and Springfield), which means the Amtrak Shuttle line needs to be electrified, double-tracked, and partially curve-modified anyway. Under the Hartford option this is not required except to provide regional service to Wallingford and Meriden, so the bypassed parts of the legacy line could be built to lower standards.
That said, 60 km of 160-200 km/h electrified track is still a lot cheaper than 60 km of 250-270 km/h track, which means that this reduces the cost advantage of I-95 but does not eliminate it. Of course 60 km of 250-270 km/h track is cheaper than 60 km of 360 km/h track, but I-95 still involves much less overall greenfield track construction.Hartford is also more compatible with any plans Rhode Island might make for southward commuter rail service. The current plans are too low-ridership to bother accommodating, but future plans might involve higher service levels.
Conversely, I-95 is useful for Shore Line East service, since regional trains could use the Quinnipiac bridge as a shortcut. The tracks cross in East Haven and a track connection could be built; it is likely that there will always be enough capacity for 5 km of track-sharing between intercity and regional trains. I-95 is also useful for the New London connection in case anyone wants to build a New London-Norwich regional train serving Mohegan Sun on the way.
Neither route is particularly expensive by the standards of what both Penn Design and Amtrak think are appropriate budgets. At French construction costs, 24 km of high-speed track with no tunnels, few viaducts, and a mostly preexisting Interstate right-of-way should be about $2.5 billion. Likewise, the cost of 167 km with only 2 km of tunnel and a fair number of viaducts should be less than $4 billion, possibly down to $3.5 billion.
However, in case there’s only enough money for part of the route, construction has to be phased. The Hartford route has no track connections to usable passenger railroads between Hartford and Providence, so the only useful partial construction there is the entire Hartford-Providence segment at once plus electrification of New Haven-Hartford(-Springfield). The I-95 route comes sufficiently close to the legacy track in East Haven and Old Saybrook, giving three segments each of which can be built separately: across the Quinnipiac, from East Haven to Old Saybrook, and from Old Saybrook to Kingston.
Every possible train station on a route deserves an answer to two questions: what is the time advantage gained by skipping it?, and who is served by it?. Stations very close to urban terminals, for example Back Bay, have a very low stop penalty because of low approach speeds, but don’t add much service since people can just ride to the urban terminal. Suburban stations such as Route 128 and even Stamford given necessary track upgrades impose high enough a cost that they should also be skipped by express trains even if there’s a fair number of people who’d use them on the local trains.
Between New York and Boston, there are three stations where the answers to both questions favor express stops: New Haven, Hartford, Providence. With New Haven and Providence, the time cost of serving them is so low given urban curves that the only way to skip them at speed is to build new urban tunnels, which cost a lot of money relative to how much money they save. With Hartford, the situation is the same if all trains go via the inland route that serves it.
However, on some level, the time cost of serving Hartford is 9 minutes, compared with about 2 for Providence. But this is not really comparable, so we can’t just say “9 minutes is too much,” as it would be if a station on a running line imposed a 9-minute stop penalty. If we skip an intermediate station that imposes a time penalty of 4 minutes, the express trains gain 4 minutes but there are still local trains serving it. In contrast, if we go via I-95 we save 9 minutes but have no way of serving Hartford on local trains; trains can branch off north of New Haven and serve Hartford and Springfield at lower speed, but this only connects Hartford to New Haven and points south rather than to Providence or Boston. So we lose something more fundamental than stopping train frequency.
So it’s not enough to say that Hartford should be skipped because it saves the trains 9 minutes. That cost-benefit calculation depends on how important serving Hartford is to people. It’s up to the potential users of Northeast Corridor HSR and the politicians providing the funding to decide whether it’s worth it to connect Hartford with Providence and Boston.