During the years Michael R. Bloomberg served as mayor, the city saw a need to create more modern office space and borrowed nearly $3 billion to extend the No. 7 line, which was essential to developing the Far West Side. The financing plan giddily anticipated that the new buildings would generate enough revenue to pay off the bonds, though the city might need to cover the interest payments for a few years. How much would the taxpayers kick in? A study done for the city by Cushman and Wakefield projected that the amount could be as little as $7 million, or in the most pessimistic version, $205 million. In fact, the bill to the public for the interest has turned out to be $359 million.

Freddi Goldstein, a spokeswoman for Mayor Bill de Blasio, said that as of this year, the project was providing enough revenue to pay the interest, and that the city expected to collect $100 million per year through at least the middle of 2021, and beyond. Officials in the de Blasio administration said they refinanced the debt in a way that would bring money to the city treasury faster.

But where will the additional estimated $96 million in costs come from?

Ultimately, from the taxpayers’ purse, city budget officials said.

While property values are climbing around Hudson Yards, no one can say with authority what is happening to commercial properties that are being vacated to fill the gleaming new spaces. Other public services, beyond the new subway station, will be needed for Hudson Yards, and the total value of tax breaks will be higher than the $723 million tallied so far, said James Parrott, a fiscal analyst who has followed the development.

Another chapter of this saga relates to our transit system. The land for the train yards was bought by the M.T.A. in the early 1980s with the idea that one day it would be valuable. The money from developing it could support the constant rebuilding of the century-old system, Mr. Ravitch said.

But that’s not what happened. Instead, it was used (along with city subsidies) to pay for new rails for the No. 7 extension.

The ancient signals on the existing subway lines are coming into their final years of service. The signals are to the subway what a pacemaker is to the heart. They control how fast things can move. With the growth of the population, that has become a critical need. “The reason you can’t move more people is the signal system,” Mr. Ravitch said.

The train yards didn’t buy us any new signals. Instead, they paid for a new stop on the No. 7 line, making possible a new office territory — and created the perfect moment to own a moving van.