Large part of subway system was built with private money, same as with railroads all over USA. NYC didn't have money then, and NYS still must borrow huge sums (which the MTA can get away with because it is a public authority) to build relatively small amounts.
Post WWII focus was on motor vehicle travel anyway, and that is where federal, state and local funding went. In contrast during same period all over Europe countries were investing heavily in rebuilding, expanding, and developing next generations of rail travel.
MTA is spending north of $11 *BILLION* USD for that boondoggle of a rat's nest project known as East Side Access.
East Side Access price tag passes $11 billion after latest increase - Second Ave. Sagas
https://www.nydailynews.com/new-york...gqu-story.html
Don't get me started on SAS, another hugely expensive project.
https://www.vox.com/policy-and-polit...subway-phase-2
https://www.nytimes.com/2017/12/28/n...ion-costs.html