Deep-level subway tunnels are usually built with tunnel-boring machines (TBMs), which can dig and create their own lining even under other infrastructure, such as older intersecting tunnels. But then deep-level stations require larger caverns, which are expensive to dig from the surface. Three-quarters of the cost of Second Avenue Subway Phase 1 is the three stations. As commenters Jim and Anon256 noted a year and a half ago, to avoid this problem, such cities as Barcelona pioneered the use of large-diameter TBMs, which have enough space to accommodate tracks together with platforms by their sides. This is especially useful for construction in dense city centers, where surface disruption must be minimized and demolitions of buildings that are in the way are expensive. I claim that this is the optimal construction method for both regional rail to Lower Manhattan and the North-South Rail Link in Boston.

In Barcelona, the internal diameter of the TBM used for Line 9, 11.7 meters, is enough to have both directions of a two-track line use one tunnel. With an internal horizontal slab, trains can be stacked so that each direction gets one track and one platform at a station, which looks about 4.5 meters wide in diagrams. Between stations, there is enough space for each of the two levels to have two tracks, allowing crossovers. The only required construction outside the tunnel is access points, which can be drilled straight down for elevators or at an angle for escalators.

While the cost of Barcelona Metro Line 9 is about $170 million per kilometer, more than three times the original budget, compared with $40-60 million per kilometer for most Spanish tunneling projects, it is still much lower than the cost of comparable projects tunneling under preexisting subway systems that have stations built by blasting caverns or cut-and-cover construction. In addition, the standards are relatively easy to adapt to the standards of American mainline construction, since the Line 9 trains are powered by catenary and are only ten centimeters shorter than the LIRR’s M-7s. Mainline catenary is energized at 25 kV and requires more clearance than low-voltage rapid transit catenary, but this adds only about half a meter to the total diameter: German standards call for 27 centimeters of clearance from 25 kV.

To allow two lines to meet at cross-platform transfers, there are two possibilities, both used by narrower-diameter TBMs (or older tunneling shields). One, used by the London Underground’s tube lines, is to have two parallel circular tunnels with numerous passages drilled between them. Another, used by some subway lines in Shanghai and Tokyo as well as by the Harlem River tunnels of New York’s Lexington Avenue Line, is to overlap the two circular tunnels, using a tunneling shield with a double-O tube (DOT) design. The DOT design is more complex and would also require any access point to either obstruct the platforms or go at the platform edges, but would create a wider platform allowing easier cross-platform circulation.

In Boston, regardless of which design is used, the North-South Rail Link involves three central stations in which two tubes (one feeding the Worcester and Providence Lines, one feeding the Fairmount and Old Colony Lines) meet: South Station, Aquarium, and North Station. Each should have a cross-platform transfer, in the style of the Hong Kong MTR: at Aquarium northbound Providence and Worcester trains should face northbound Fairmount and Old Colony trains and likewise for southbound trains, whereas at South and North Stations, northbound trains should face southbound trains. This way, people transferring between two points south of the link could transfer cross-platform at South Station, and people transferring between two points north of the link could transfer cross-platform at North Station.

A large-diameter TBM has enough space not only for crossovers, but for trains to switch what levels they’re on. With a design speed of 100 km/h, a curve radius of 500 meters, and a superelevation ramp lasting 2 seconds, it takes about half a kilometer for the track on the lower level to swerve sideways so as to no longer be directly under the upper-level track, climb to the upper level while the upper-level track descends, and then swerve sideways again so that both tracks are on the correct side of the tunnel to allow a cross-platfom transfer. There is space to do this between both pairs of successive stations. The portals could be constructed where convenient on the approaches to South Station and immediately north of the Charles, and the infrastructure for pairing lines at the north end with the two tubes could be done above or below ground, based on local tradeoffs between disruption and cost.

In Lower Manhattan, the problem is capacity. The system would involve a line from Atlantic Terminal to Jersey City or Hoboken intersecting a line from Grand Central to Staten Island. There is room for only one station, and some configurations, notably any in which the New Jersey end is at Exchange Place, require a cruciform station, without cross-platform transfers. Moreover, this station is at a site with much more intensive development than Downtown Boston, and close attention must be paid to capacity. This is why I bring up DOTs in the first place: London-style passages may not allow sufficient circulation of transferring passengers. The platforms would be obstructed with many escalators between the upper and lower levels since there is no room for Hong Kong’s three-station cross-platform transfers, and peak demand for egress to both street level and intersecting subways is also likely to be very high.

The optimal solution seems to be to have no real Lower Manhattan station beyond the platforms and access points. Most ticket-vending machines should be placed at street level next to the escalator and elevator banks, and the blocks above the station should be pedestrianized to allow for access from the middle of the street, avoiding the need for a mezzanine. The width and pedestrian volume of Lower Manhattan streets are such that it would be at good human scale.

The remaining capacity issue is sufficient space for escalators. There are four tracks in total, each of which is inbound from some direction, and at the peak there could be a 12-car, 300-meter long train with 2,000 passengers every 2 minutes per track. If all passengers are discharged and the trains leave the station empty in the morning peak, then the required capacity is 240,000 people per hour. This is in fact quite unlikely, even though there is only one Lower Manhattan stop: many Staten Islanders work in Brooklyn or Midtown, people from points north of Grand Central are more likely to get off the train at Grand Central than to stay on until Lower Manhattan, and there is a substantial volume of commuters between Brooklyn and points west or north of Manhattan, who would benefit the most from through running.

Factsheets by Kone and ThyssenKrupp suggest each meter-wide escalator has a practical capacity of 6,000-7,000 passengers per hour. If we assume half of a full train capacity’s worth of passengers get off at the station, not including passengers who transfer, then we need 120,000 passengers per hour, i.e. seventeen to twenty escalators. This can be done quite easily with two parallel circular bores, at the cost of restricted capacity for connecting passengers. With a DOT design with 8-meter wide platforms, it’s still possible to have an escalator bank at each end of each platform; the large separation between the upper and lower levels, about 6 meters, allows independent escalators at the end, though not anywhere else. The widest standard escalator is a meter wide at the step and requires a 1.6-meter wide pit (see above ThyssenKrupp link as well as brochures by Kone and Otis), enough for a three-and-one or three-and-two escalator bank at each end, giving twelve peak-direction escalators. Eight additional escalator banks in a one-and-one configuration (or perhaps four in a two-and-one configuration, which is a wider platform obstruction) can be placed roughly evenly along the upper-level platform, along with elevator shafts, escalators that only connect the two platforms, and access points to intersecting subway lines.

The advantage in both New York and Boston is that there’s no need to construct a station beyond those shafts and bores. The station mezzanine in this configuration is a street, most likely Broadway in Lower Manhattan and (according to prior North-South Rail Link plans) the greenway above the Central Artery tunnel in Boston. The station retail is ordinary street retail. Fare control is roving inspectors riding the trains or patrolling the platforms. It’s still a multi-billion dollar undertaking due to all the underwater access tunnels, but the cost per kilometer could be held down to normal first-world levels even while crossing the difficult infrastructure of Lower Manhattan and Downtown Boston.

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.