Let me preface this post by saying that there should not be any high-speed trains between New York and New Rochelle, except perhaps right at the northern end of the segment. However, to provide reasonable speeds from New York to Boston, it’s desirable to upgrade the maximum speed between New York and New Rochelle to 200 km/h or not much less. The subject of this post is how this can be accommodated while also permitting some regional rail service, as proposed by the MTA. There are two reasons to bundle the two. First, some of the work required could be shared: for example, new stations could be done at the same time as rail and tie replacement. And second, the presence of both upgraded intercity rail and regional rail on the line requires some four-tracking and schedule optimization.
The physical infrastructure required for boosting speeds within New York City is fairly minimal by itself. The right-of-way in the Bronx has some curves but they are not very sharp and can be somewhat straightened without knocking down buildings, and even the curves in Queens and on the Hell Gate Bridge, while unfixable without major viaduct modification, are not terrible if superelevation is high and tilting is enabled.
A big question mark is what the maximum speed permitted by the physical layout of the East River Tunnels is. Current speed is 97 km/h (60 mph), but top speed today in other sections of the network are below those achieved decades ago (for example on Portal Bridge), and trains with specially designed noses, as the Shinkansen rolling stock is, could potentially go even faster. Regardless, it is not important for HSR-regional rail integration, since the East River Tunnels have no stops and will be running far under capacity once East Side Access opens. Thus, all travel times in this post are between New Rochelle and Sunnyside Junction, which is notionally considered to be located at 39th Street. This is a 25-kilometer segment.
Another question mark is what the speed limit on the S-curve south of New Rochelle is. Currently the limit is 48 km/h (30 mph). Raising it requires grade-separating the junction between the NEC and the current New Haven Line. It can be raised further via curve straightening, but the question is how much eminent domain can be done. The maximum radius that can be achieved with minimal or no eminent domain is 700-800 meters. Some further eminent domain may be required to have this curve start far enough from the southbound platform that full 200 mm superelevation is achievable without subjecting local train riders to too much cant excess. For comparison, slicing through New Rochelle and the Pelham Country Club allows essentially eliminating the curve and allowing maximum speed through the area, which taking surrounding curves into consideration is about 240 km/h.
Assuming 150 km/h (about 700 meters radius, 200 mm cant, and 175 mm cant deficiency), the technical travel time for a nonstop intercity train between when it passes New Rochelle and when it passes Sunnyside is about 9 minutes; this includes slowdowns in Queens and the Bronx and on Hell Gate. A nonstop M8 with a top speed of 145 km/h would do the same trip in about 11:15. (Amtrak’s current travel time from New York to New Rochelle is about 25 minutes, of which by my observation riding Regional trains 6 are south of Sunnyside.)
Even the above travel time figures require some four-tracking, independently of capacity, in order to limit cant excess. Unlike the Providence Line, the Hell Gate line has some curves right at potential station locations – for example, the Hunts Point stop is located very close to the curve around the Bruckner Expressway, and the Morris Park stop is located in the middle of a curve. The Bruckner curve radius is about 500 meters, and 200 mm superelevation would impose 80 mm cant excess on even a fast-accelerating commuter train (1 m/s^2 to 72 km/h), and an uncomfortable 140 mm on a slower-accelerating one (0.5 m/s^2 to 51 km/h). The Morris Park curve is even worse, since it would impose a full 200 mm cant excess on a stopped train. So we should assume four-tracking at least at the Morris Park station, which is located in the middle of a curve, and Hunts Points, and potentially also at Parkchester.
Now, a local train would be stopping at New Rochelle and four stops in the Bronx, and should be stopping at Sunnyside. Although a FLIRT loses only about 75 seconds from a stop in 160 km/h territory, assuming 30-second dwell times, the M8 is a heavier, slower-accelerating train, and for our purposes we should assume a 90-second stop penalty. This means that, counting New Rochelle and Sunnyside together as a single dwell-free stop (they involve one acceleration and one deceleration in the Sunnyside-New Rochelle segment), local technical travel time is 18:15, about the same as what Amtrak achieves today without stops but with less superelevaiton and inferior rolling stock.
Now, 18:15-9 = 9:15, 9:15 times the schedule pad factor is 9:54, and modern signaling allows 2-minute headways up to 200 km/h; thus we can accommodate 4 tph intercity and 4 tph local Metro-North without overtakes except at New Rochelle and Sunnyside.
There is only one problem with the no-overtake scenario: the MTA plans on a peak traffic much higher than 4 tph, in line with the New Haven Line’s high demand. It’s planning on a peak of 6-8 tph according to what I’ve read in comments on Second Avenue Sagas. This naturally breaks into 4 tph that make local stops and 4 that do not (though my suspicion of MTA practice is that it wants fewer than 4 local tph); if there are fewer than 8 trains, one slot could be eliminated.
Let’s look then at a 4/4/4 scenario. Assume that trains depart Sunnyside in order of speed – HSR first (passing rather than stopping at Sunnyside), then express Metro-North, then local Metro-North. A local train will be overtaken first by the following HSR, and then by the following express. If we could move the overtake point to New Rochelle, the local would not need to wait for trains to pass it. In reality, 4/4/4 means the local departs Sunnyside 4-5 minutes after the HSR train passes it, and has 9 minutes of time penalty before being overtaken again. If the stop penalty could be reduced to 75 seconds, then the overtake could be moved to New Rochelle, demonstrating the use of top-quality rolling stock. But the M8s are good enough for many purposes, and therefore we will not assume a noncompliant replacement, unlike in the case of the MBTA, whose rolling stock is slow and very heavy.
With 9 minutes of time to make up, it’s tempting to have an overtake at a four-tracked Co-op City station. But then the local would have to be overtaken by two trains in a row, and moreover the two trains would become quite separated by then due to differing top speeds, and this would force a penalty on the order of 6 minutes.
I claim that the best would be to four-track a segment between two or even three stations; the right-of-way is wide enough anyway. In addition, the Morris Park curve could be straightened if the Eastchester Avenue overpass were modified, and doing this in conjunction with four-tracking would be cheaper than doing each alone. Under this option, the local would leave Sunnyside much later than 2 minutes after the express, just enough to be overtaken by HSR at Morris Park. It would then keep going to Co-op City until overtaken by an express. This would essentially save about 2.5 minutes out of the 6 in penalty, since the train would be in motion for that time.