> Dig deeper

The Vox article [1] specifically cites depth as a contributor to the Second Avenue Line's increased cost:

"the majority of the Second Avenue subway’s price tag actually comes from the stations rather than the tunnels. The line is being built unusually deep for a subway tunnel, which doesn’t do much to change the basic logistics of boring a tunnel, but does make excavation of station caverns unusually difficult."

[1] http://www.vox.com/policy-and-politics/2017/1/1/14112776/new...

Geology and hydrology are enormously different between those locations. Not saying you definitely can't do that, since I'm no expert either, but it's kinda silly to just say "dig deeper" like it's a no-brainer.

2nd avenue subway is muuuuch deeper than most of the 100 year old stations in manhattan, for just this very reason. And also because no one would stand for cut and cover anymore.

"Cut and cover" being what exactly? Trenching down from the surface and then filling in above the tracks? Hardly seems practical beyond a fairly shallow depth.

Yet with the exception of London, that's how most of the world's pre-existing subways were built. London is a bit of an outlier as it was very highly developed even in the late 19th century and there was a great fear of damaging existing "tall" buildings. Although even there a large amount of cut and cover tunneling exists.

What happens when you build the next infrastructure project? Then you have to dig even deeper to be beneath this one.