I don't understand. That's not the tradeoff MTA is making. They're building more capacity now rather than having to try to do it later.

Later would be nigh-impossible because of the skyscrapers going up on every side, and the railyard above. Hudson Yards is a massively complex project.

The parent comment that you responded to:

"... Oversized stations are even more problematic when you consider that the MTA wants to build all of these by mining rather than using cut-and-cover, because cut-and-cover has too much disruption to surface tenants. And mining is far more expensive than cut-and-cover..."

Your response:

"... It is ALWAYS cheaper to build capacity now than to retrofit later, even inflation adjusted. ..."

The implication of your response is that on a 100-year timeline, the way that the modern stations are apparently being built (by mining) is cheaper than building them by the way the parent commenter says (cut-and-cover).

I was just curious why it's "always" cheaper. Is "mine and build big" the cheaper method only when there's a ton of density on top like Hudson Yards, or is it also the case in places with less surface density?

I think that you’re probably always better off building bigger if you expect growth in the next 100 years. How big you should build is just a matter of how much current and future traffic you expect to have.

Apologies - I may have skipped over something when reading. I was responding to the implication that a smaller station now is an appropriate cost savings.

I think what they're looking at is over-sizing rather than right-sizing. Think of it as cost=size*method. Future-proofing increases the size, not the method.

Capacity for what? The line could theoretically be extended by a tunnel to Secaucus but even then only a fraction of the ridership will take that branch and get off at the first stop. The West side is already serviced by two lines so where else can the 7 go?