I hate the term “apples-to-apples.” I’ve heard those exact three words from so many senior people at or near New York subway construction in response to any cost comparison. Per those people, it’s inconceivable that if New York builds subways for $2 billion/km, other cities could do it for $200 million/km. Or, once they’ve been convinced that those are the right costs, there must be some justifiable reason – New York must be a uniquely difficult tunneling environment, or its size must mean it needs to build bigger stations and tunnels, or it must have more complex utilities than other cities, or it must be harder to tunnel in an old, dense industrial metropolis. Sometimes the excuses are more institutional but always drawn to exculpate the political appointees and senior management – health benefits are a popular excuse and so is a line like “we care about worker rights/disability rights in America.” The excuses vary but there’s always something. All of these excuses can be individually disposed of fairly easily – for example, the line about worker and disability rights is painful when one looks at the construction costs in the Nordic countries. But instead of rehashing this, it’s valuable to look at some ways in which New York is an easier tunneling environment than many comparison cases.
New York does not have active seismology. The earthquake-proofing required in such cities as Los Angeles, San Francisco, Tokyo, Istanbul, and Naples can be skipped; this means that simpler construction techniques are viable.
Nor is New York in an alluvial floodplain. The hard schist of Manhattan is not the best rock to tunnel in (not because it’s hard – gneiss is hard and great to tunnel in – but because it’s brittle), but cut-and-cover is viable. The ground is not going to sink 30 cm from subway construction as it did in Amsterdam – the hard rock can hold with limited building subsidence.
The underwater crossings are unusually long, but they are not unusually deep. Marmaray and the Transbay Tube both had to go under deep channels; no proposed East River or Hudson crossing has to be nearly so deep, and conventional tunnel boring is unproblematic.
History and archeology
In the United Kingdom, 200 miles is a long way. In the United States, 200 years is a long time. New York is an old historic city by American standards and by industrial standards, but it is not an old historic city by any European or Asian standard, unless the standard in question is that of Dubai. There are no priceless monuments in its underground, unlike those uncovered during tunneling in Mexico City, Istanbul, Rome, or Athens; the last three have tunneled through areas with urban history going back to Classical Antiquity.
In addition to past archeological artifacts, very old cities also run into the issue of priceless ruins. Rome Metro Line C’s ongoing expansion is unusually expensive for Italy – segment T3 is $490 million per km in PPP 2022 dollars – because it passes by the Imperial Forum and the Colosseum, where no expense can be spared in protecting monuments from destruction by building subsidence, limited by law to 3 mm; the stations are deep-mined because cut-and-cover is too destructive and so is the Barcelona method of large-diameter bores. More typical recent tunnels in Rome and Milan, even with the extra costs of archeology and earthquake-proofing, are $150-300 million/km (Rome costing more than Milan).
In New York, in contrast, buildings are valued for commercial purposes, not historic purposes. Moreover, in the neighborhoods where subways are built or should be, there is extensive transit-oriented development opportunity near the stations, where the subsidence risk is the greatest. It’s possible to be more tolerant of risk to buildings in such an environment; in contrast, New York spent effort shoring up a building on Second Avenue that is now being replaced with a bigger building for TOD anyway.
New York is a city of straight, wide streets. A 25-meter avenue is considered narrow; 30 is more typical. This is sufficient for cut-and-cover without complications – indeed, it was sufficient for four-track cut-and-cover in the 1900s. Bored tunnels can go underneath those same streets without running into building foundations and therefore do not need to be very deep unless they undercross older subway lines.
Moreover, the city’s grid makes it easier to shut down traffic on a street during construction. If Second Avenue is not viable as a through-route during construction, the city can make First Avenue two-way for the duration. Few streets are truly irreplaceable, even outside Manhattan, where the grid has more interruptions. For example, if an eastward extension of the F train under Hillside is desired, Jamaica can substitute for Hillside during construction and this makes the cut-and-cover pain (even if just at stations) more manageable.
The straightforward grid also makes station construction easier. There is no need to find staging grounds for stations such as public parks when there’s a wide street that can be shut down for construction. It’s also simple to build exits onto sidewalks or street medians to provide rapid egress in all directions from the platform.
Older infrastructure, in isolation, makes it difficult to build new tunnels, and New York has it in droves. But things are rarely isolated. It matters what older infrastructure is available, and sometimes it’s a boon more than a bane.
One way it can be a boon is if older construction made provisions for future expansion. This is the most common in cities with long histories of unrealized plans, or else the future expansion would have been done already; worldwide, the top two cities in such are New York and Berlin. The track map of the subway is full of little bellmouths and provisions for crossing stations, many at locations that are not at all useful today but many others at locations that are. Want to extend the subway to Kings Plaza under Utica? You’re in luck, there’s already a bellmouth leading from the station on the 3/4 trains. How about going to Sheepshead Bay on Nostrand? You’re in luck again, trackways leading past the current 2/5 terminus at Flatbush Avenue exist as the station was intended to be only a temporary terminal.
Second Avenue Subway Phase 2 also benefits from such older infrastructure – cut-and-cover tunnels between the stations preexist and will be reused, so only the stations need to be built and the harder segment curving under 125th Street crossing under the 4/5/6.