The most straightforward part of constructing greenfield tracks for high-speed rail on the Northeast Corridor is east of New Haven. There are good legacy lines to hook into, and good Interstate corridors to follow when the legacy lines are too curvy. It’s also the segment with the biggest variation in alignment options, which boil down to going through Hartford and going through Providence. Both the Penn design proposal and the Amtrak proposal go through Hartford and avoid Providence, and this is a bad idea, for both costs and benefits reasons.
See here for a very early and rough draft of my HSR map, which goes through Providence; there are significant issues with this map west of New Haven, but it’s fairly accurate east of Haven. It uses I-95 between New Haven and the state line, and transitions to the legacy line around Kingston; Hartford would be served on the legacy line, which would be electrified. I have not seen detailed drawings of Amtrak’s proposal, but here is the detailed Penn design map, going through Hartford: the idea is to use a combination of I-91 and a heavily upgraded legacy line, and then transition to I-84 in Hartford and then I-90, while retaining the Shore Line for slower service to Providence. The latter option turns out to be inferior, essentially because full HSR is easier to build through Providence whereas a medium-speed branch is easier to build to Hartford.
First, the cost side. Because the portions of the Shore Line used by the Providence option are straight and already built to high standards, minimal upgrade work is required there. The bulk of the cost would be constructing high-speed track along a mostly flat, not very developed right-of-way, with two and a half painful segments (New London, the cutoff east of New Haven, and the Connecticut River crossing as the half). East of New London the median is available, cutting costs further. All in all, this is 125 km of largely at-grade track, and about 60 km of cheap electrification to Hartford.
Going through Hartford is about equally hard. The New Haven-Springfield line is built to low-speed standards, with grade crossings and curves that are good for 200 km/h rather than 300. It avoids the river crossings of I-95, but I-84 and I-90 are a bit curvier and follow more rugged and urbanized terrain, and the urban segment through Hartford looks harder than that through and immediately east of New Haven. Per kilometer it could cost about the same, but 200 km of new track are required.
The costs by themselves are not a huge deal. The New York-New Haven segment requires new grade-separated junctions, multiple bypasses, and some urban tunneling. In contrast, mostly at-grade track costs $20 million per kilometer or not much more, so despite the large difference in length, the difference in cost is about $2.5 billion vs. $4 billion.
However, there’s also a constructability argument for I-95, which is that it can be done in segments more easily, using portions of the Shore Line before the full line opens. This could be useful if money were made available in very small chunks. The Hartford route could be done partly on an electrified Springfield line, but Hartford-Boston has to be done in one go.
But a bigger issue is that going through Providence has two advantages over going through Hartford without regards to costs. First, Providence is a larger city than Hartford: its metro area is about 20% larger than Hartford’s, and the central city is 40% larger and denser. Although the Hartford option passes near Worcester, there is no way to bring a station into Worcester itself without excessive tunneling; the Penn design plan puts the station at the edge of the built-up area, 7 kilometers from downtown Worcester. The Providence option passes through much smaller New London, but it can at least be served by a station that’s within the city, one km from the present station.
The other advantage is how to serve the city that does not get to be on the HSR mainline. The Springfield line is easy to upgrade, since it is straight enough for medium speed, and grade crossing protection good for about 180 km/h is relatively cheap. This would give Hartford very good service to New York – about half an hour to New Haven, and a little more than another half an hour to New York. The Shore Line in contrast is curvy and slow and already has a fair amount of superelevation and cant deficiency, making future upgrades much harder. Providence would still get better trip times than today coming from better rolling stock and higher speeds west of New Haven, but better trip times than about 1:45 to New York are only possible with trains with high degree of tilt, which tend to be a maintenance nightmare.
For the record, my original proposal above is from 2009, and I only accepted my current job at Brown in 2011. However, in the interest of full disclosure, by 2009 I already knew that Brown had one of the best departments in my field, whereas Hartford doesn’t have a research university of comparable quality. I don’t think it biased my choice – the idea of following the present alignments and serving present lines as much as possible appears elsewhere in the plan as well as in my regional rail proposals for New York and Boston – but then again nobody thinks their own choices are biased.