This post responds to arguments made by Brian in comments regarding how to connect New Jersey regional trains to Manhattan, in addition to the present tunnels to Penn Station; Brian argues for leveraging the Staten Island Railway, including the North Shore Branch, since a Staten Island-Manhattan tunnel should be built anyway.
In my post about the various options for connecting New Jersey to Lower Manhattan, all four alternatives I looked at featured a tunnel across the Hudson from the Hudson County waterfront to Manhattan, differing only in the location of the portals and the route used to get to the New Jersey portal. There are in principle other options, and I’d like to explain why they’re less feasible, and conversely why a connection along the lines I suggested should be one of the top two priority trans-Hudson projects, together with an additional tunnel pair to Penn Station.
First, because Lower Manhattan is the second most important business district in the region, as well as a subway hub, it deserves some connection. More than that, it deserves a connection from as many directions as possible, same as Midtown, and it deserves a connection earlier rather than later. The longer it takes to build a direct commuter rail line to it, the more it will decline in favor of other business districts, which with the exception of Midtown are much harder to serve with transit. It’s likely that if the LIRR, the Pennyslvania, the Lackawanna, the Erie, and the New York Central had all managed to build commuter lines to Lower Manhattan, instead of relying on the subway and the Hudson Tubes for the final connection, Lower Manhattan would not have lost out to Midtown so readily; Midtown would remain more convenient for commuters from Uptown Manhattan, the Bronx, and Queens, but not for commuters from Long Island or New Jersey.
Because of those principles, we get that a connection from the Erie lines to Lower Manhattan is critical. Once we accept that the major New Jersey lines, or groups of lines, need to be connected to both Manhattan job centers, it becomes best to gear the Lower Manhattan connection to the Erie lines, which are the northernmost in New Jersey and therefore wouldn’t intersect a Lower Manhattan connection to another line. The ARC solution of looping trains around Secaucus and connecting them to Penn Station is a fine first step but is inadequate afterward: a Lower Manhattan connection from the Erie lines would intersect the other lines at Secaucus, allowing a transfer, but a connection from any other direction would not allow a transfer from the Erie lines to Lower Manhattan.
On top of this, the cost involved in building such a connection, along any of the four alignments I proposed, is a tunnel across the Hudson, some extra tunneling on the Manhattan or Jersey City side (the farther south the alignment, the more Jersey City and the less Manhattan tunneling is needed), and of course a station in Lower Manhattan. This is quite bare-bones in the sense that any other connection to Lower Manhattan has to incur the same costs of a tunnel across water, and a Manhattan station. Concretely, this means it’s easier to tunnel from Jersey City or Hoboken to Manhattan than from Staten Island to Manhattan, and as such this would be built first, becoming the initial connection from New Jersey to Lower Manhattan.
I waver on whether this should be done before or after four-tracking the North River Tunnels. The tunnels are still extraordinarily busy at rush hour, and even state of the art signaling will only buy a few years before traffic matches the new capacity; moreover, Lower Manhattan-bound commuters can already transfer to PATH at Newark Penn cross-platform or at Hoboken, either of which is more convenient than transferring at Penn Station. On the other hand, people can also get to the southern edge of Midtown on PATH, and direct Lower Manhattan service can justify diverting some Morris and Essex trains from the mainline. It buys at most a few more years of breathing room, but it adds more destinations that can be reached by train, whereas a Midtown solution just adds capacity to an existing destination.
But, now, what of a future Staten Island connection? If a Staten Island-Manhattan tunnel is built, along the straightest alignment, bypassing Brooklyn, then it could provide a second connection from New Jersey to Lower Manhattan. This is the brunt of Brian’s comment: it would require using the bridge from Elizabeth to the North Shore Branch, which is active, and for another access point a new bridge from the mainline to Perth Amboy, but even building the latter bridge costs much less than new tunnels. Here is a map of the alignments.
The problem with using this for through-trains from the Jersey Shore and the Raritan Valley Line, the lines that connect best to Staten Island, is speed. The distance to Grand Central through either Staten Island and Lower Manhattan or the Northeast Corridor and Penn Station is about the same; the distance to Lower Manhattan is several kilometers shorter and one transfer fewer than via Secaucus, but once one connection to Lower Manhattan exists, a secondary connection would have to be justified based on demand to all job centers, of which Midtown is the biggest.
But now the Staten Island connection would have a much lower average speed. It is curvier, independently of all other considerations. The tunnel from Staten Island to Manhattan should also be lower-speed, to reduce the required bore diameter and save money. Since there is no good reason for intercity trains to use this connection – the Perth Amboy connection leads to no intercity line, and the North Shore Branch connection would require building a new junction to the Northeast Corridor, which would be both expensive and curvy – there is no reason to optimize for speed, unlike the case for the Northeast Corridor. So the choice is between one line where express commuter trains could do 160 km/h except maybe in the last few kilometers into Manhattan, and one where they’d do 100 or charitably 130.
On top of that, there are more stations in Staten Island, and also more local demand. Part of it is just bad operating practices in New Jersey – there should be more local stops in Elizabeth – but Staten Island has far more local demand, and so dropping local stops to make it easier to run express trains is less justified. As of 2000, the latest year for which the census data is readily available, Staten Island had 53,000 Manhattan-bound commuters. The relevant intermediate cities on the Northeast Corridor and North Jersey Coast Line – Newark, Elizabeth, Linden, Rahway, Carteret, and Woodbridge – had 10,500 between them. The corresponding numbers of Brooklyn-bound commuters are 29,000 and 1,500, respectively. It makes sense to keep the current stop spacing on the trunk line between Newark and Rahway, or add just one or two stops, but it makes none to not fit a North Shore Branch service with many local stops, which would then slow down longer-distance regional trains.
While the North Shore Branch can’t be widened except with many takings, the Staten Island Railway mainline could conceivably be four-tracked to allow overtakes, and this would make it a more competitive route. But if there is money for that, there is probably money to six-track the remaining four-track gap between Newark Airport and Linden, allowing full separation of local commuter trains, express commuter trains, and intercity trains on the Northeast Corridor except for segments on which the speeds are similar (Newark-New York) or ones where traffic is low enough to fit on existing tracks (south of Rahway).
The problem is really that the North Jersey Coast Line doesn’t have enough traffic to justify two highly separated branches, one through Staten Island and one through the Northeast Corridor. The split I proposed in my regional rail posts is much smaller – trains are only split east of Penn Station, after they begin overlapping with the Morris and Essex Lines, and so it’s possible to time transfers in such a way that people from Long Branch can board any train and be at their destination with just one additional easy transfer. At most this may justify a few peak hour runs; otherwise, even if the Tottenville-Perth Amboy bridge is built, timed transfers at Perth Amboy are almost as good and avoid reducing frequency on each branch too much.