Midtown’s Hammerstein Ballroom used to be an opera house. Thursday, Gov. Andrew Cuomo showed up there to act out his favorite part, the hero. He declared New York’s subway system to be in “a state of emergency.” Cuomo promised to rescue New Yorkers from the MTA’s “deplorable,” “tortured” system. It’s still too early to tell whether this opera will be a comedy or a tragedy — with the 5.6 million people who ride the subways every day starring as the victims.

Yes, we know already — New York has grown and the subway system hasn’t kept up, blah blah.

But here are a couple of newer statistics. For the first five months of this year, only 65 percent of trains were on time, a decline from last year’s 69 percent, and a huge drop from the 86 percent that was normal five years ago.

Trains are failing more often — every 115,527 miles, down from 170,206, or 32 percent, from half a decade ago. In April 2012, 21,944 trains suffered delays. By April 2017, the figure was 58,651.

It makes a huge difference in people’s lives if their train to Ocean Parkway or Dyckman Street is supposed to get there in 40 minutes, and it regularly takes more than an hour — but you don’t know when it’s going to be running or not.

As Cuomo said Thursday, “They tweet nasty things about me all day, the riders.” (No, we’re pretty nice. If you want to see nasty, see what people say about Chris Christie — who really killed his state’s transit system.)

So what’s he going to do?

Thursday, he promised a billion dollars. It’s always nicer to have a billion dollars than not to have a billion dollars.

But this isn’t new spending. It was approved last year, as part of the MTA’s plan to invest $32.9 billion in its infrastructure. Cuomo is just throwing out a big number, and hoping some people are impressed by it.

But the MTA is still set to borrow nearly $10 billion in the next few years. The state should be putting much more of its surplus money from fines levied on big banks for money laundering — at least another billion — to cut this borrowing, as the Empire Center’s E.J. McMahon has suggested.

The MTA needs so much money partly because it takes the MTA too long, and costs too much, to build anything. The MTA needs a new signal system. But its main contract just for the No. 7 line — only the second subway line it’s done — is taking a decade, is two years late, and is $140 million over its original $266 million budget. Multiply that by 20 more subway lines, and you have big (or bigger) problems.

So what’s Cuomo going to do about that?

Thursday, he convened a panel of global experts to tell us how we can build a modern signal system more quickly and cheaply.

Of course, the MTA should learn from the rest of the world. But yesterday’s lesson was . . . remedial.

Why should New York fly a transit chief in from Toronto to tell us that we should “have people with the right skill sets in the right job” and that “you have to have a coordinated plan”? Why didn’t we think of that ourselves?

Global transit chiefs all seemed pleasant and competent. But they didn’t say much that should be useful to high-level MTA execs. If it is useful, then we are really way behind. The woman who runs the Paris Metro seemed confused as to why this should be difficult, noting that Paris modernized the signals on a metro line in just four years, working only for 3½ hours a night.

The day culminated in a repeat of Cuomo’s call to “geniuses” from the tech world to come up with ways to do this work faster and better, in return for three $1 million prizes, to be given out this fall.

True, the major accomplishment of the nation’s leading transportation-tech “geniuses” is hardly inspiring. Uber has created a loss-making industry out of an activity — driving people around for money — that had been profitable since the invention of the horse.

Still, fresh eyes might help. As Cuomo said somewhat cryptically, “The box is broken.”

But it will remain so for a while. The MTA now has 30 days, on Cuomo’s orders, to “reorganize” itself, and 60 days to come up with a better plan for how it builds and replaces its equipment.

But lots of what’s needed is basic stuff. For example, the MTA is short 65 signal maintainers out of a crew of 1,526. The MTA is short, too, 79 subway-infrastructure repair workers.

This was management failing to plan for a predictable problem. Crowding would inevitably cause enough strain that even a small glitch turns into a big delay that impacts tens of thousands of people.

That problem isn’t a lack of money.

“Part of the problem definitely is the amount of maintenance that goes on overnight and the pressures to get the work done and clear out before the rush,” one union official said. “Manpower shortage is also a part of it. Most practices haven’t changed in decades. I think this review of the system is probably a good thing.”

Meanwhile, though, Cuomo has also spoken, with his silence, on what the MTA is not to do.

He doesn’t want the agency rethinking its capital priorities.

But the MTA board, now headed by public-sector veteran Joe Lhota, should hold off on Cuomo’s recently added $2 billion project to add a third track to the Long Island Rail Road. That should wait until we figure out subway signals.

Nevertheless, Thursday’s crisis conference was useful. Cuomo vowed, publicly, that he is responsible.

“The current state of decline is wholly unacceptable,” he said. “We’re going to do something and we’re going to do something about it now.”

Good. As Michael Sciaraffo, who was stuck on the sweltering F train for over an hour earlier this month, says, “If the governor cannot do his job and fix our crumbling subway system, then next year’s gubernatorial race should and, in my opinion, will be a referendum on his re-election. And no one needs a million dollars to figure that out.”

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.