I am only loosely following the news about the second phase of Second Avenue Subway. The project, running from 96th Street to 125th, with a short segment under 125th to Lexington, passing under the 4, 5, and 6 trains, is supposed to be cheap. In the 1970s, work began on Second Avenue Subway before the city went bankrupt, and there are extant tunnel segments built cut-and-cover in East Harlem between the station sites. The stations need to be dug, but the plan dating back to 2003 was to build them cut-and-cover as well, with local disruption for only a few blocks around 106th, 116th, and 125th Streets. Only one part would be difficult: going deep under 125th, under the preexisting subway. And yet, costs are very high, and the design seems to be taking a wrong turn.

In the early 2000s, the cost projections were $3.7 billion for phase 1 (actual cost: $5 billion, but much of the difference is inflation), and $3.3 billion for phase 2 (projected cost: at least $6 billion). Since then, there have been changes. For about a year I heard rumors that the preliminary engineering had been done wrong, and it was impossible to use the preexisting tunnel segments. Then I heard that no, it’s actually possible to use the existing tunnels. But a few days ago I heard that even though it’s possible, the MTA is now planning to demolish the existing tunnels and build the entire project deep underground using tunnel-boring machines.

With the information generally given out at community meetings, it’s hard to know what’s exactly going on. However, the fact that the MTA is talking about this suggests extreme disinterest in cost control. Cut-and-cover construction is cheaper than TBMs, per a 1994 paper looking at French urban rail costs since the 1970s. The tradeoff is that it forces rail lines to go underneath streets, which is disruptive to pedestrians and merchants, or demolish private property. Fortunately, Second Avenue is a wide, straight throughfare, and requires no such demolitions, while the disruption would be localized to areas that are scheduled to get subway stops as part of the project. Metro extensions here and in a number of other European cities are constructing stations cut-and-cover and the tunnels between them with TBMs; Metro Line 12’s online documents state that station construction involves just 18 months of digging.

It’s possible that the need to turn to 125th Street is messing up the plan to do everything cut-and-cover. While the turn itself can be done with minimal demolitions (the inside of the curve has a few small buildings, and there’s also an alignment slightly farther east that goes under vacant land while maintaining a 90-meter curve radius), going underneath the Lexington Avenue Line requires diving deep, and then there is no advantage to cut-and-cover. Building cut-and-cover under existing lines is difficult, and in that case, TBMs are warranted.

If the problem is 125th Street, then I would propose extending phase 2 and then breaking it apart into two subphases. Phase 2.0 would be cut-and-cover and open stations at 106th, 116th, and possibly 125th and 2nd temporarily. Phase 2.5 would involve driving TBMs under 125th Street all the way to Broadway; this could be done with a large-diameter TBM, with the platforms contained within the bore and vertical access dug so as to avoid the intersecting north-south subways. 125th Street has 30,000 crosstown bus boardings according to the MTA, which would make it the busiest bus corridor in the city per km: 10,000 per km, compared with 8,000 on the busiest single route, the M86. It is a priority for subway expansion, and if it’s for some reason not possible to easily build from 96th and 2nd to 125th and Lex in one go then the entire project should be extended to 125th and Broadway, at somewhat higher cost and far higher benefits.

The reason phase 1 was so expensive is that the stations were mined from small digs, rather than built cut-and-cover as is more usual. The idea was to limit street disruption; instead there was street disruption lasting 5 years rather than 1.5, just at small bore sites at 72nd and 86th rather than throughout the station boxes’ footprints. The TBM drive and systems cost together $260 million per kilometer, compared with $125 million on Paris’s Metro Line 1 extension, but the stations cost $750 million each, compared with $110 million.

It’s crucial that the MTA not repeat this mistake in phase 2, and it’s crucial that area transit activists hold the MTA’s feet to the fire and demand sharp cost control. Even taking the existing premiums as a given, cut-and-cover stations should not cost more than $200 million each, which means phase 2 as planned should cost $600 million for stations, about $330 million for systems, and another $350 million for overheads. At $1.3 billion this still represents high cost per kilometer, about $500 million, but it’s based on actual New York cost items, which means it’s plausible today. There is no excuse for $6 billion.