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.