President Trump should have been paying close attention to the biggest economic crisis of his term so far.

Last week, Amtrak, the federal railroad, disrupted commutes for the 210,339 people who depend on Penn Station to get to work from New Jersey and Long Island. Trump says he wants to fix America’s infrastructure — but time is already running out.

There are many to blame for last week’s disaster. Amtrak let “weak timbers” underneath a rail deteriorate so badly, they led to a New Jersey Transit train crash last Monday morning — the second derailment in 10 days.

Because the crash happened near a switch, it knocked out a third of Penn’s capacity. Amtrak blew its deadline for fixing everything by Friday — prompting New York’s LIRR to send out a 5 a.m. email canceling 10 more trains.

Penn Station is an unforgiving space for mistakes. With New York adding jobs faster than New Jersey, more people are coming into Manhattan — with ridership up 74 percent in the past 20 years.

“Penn itself is a huge chokepoint,” says Amtrak chief Wick Moorman, whose previous experience was in the private-freight railroad business. “This derailment of three cars that all stayed inline and upright” and still knocked out service “just shows you how vulnerable this place is.”

Inspectors misjudged how long the timbers, perhaps 20 years old, would hold out — not a comforting notion. Wick notes that Amtrak “inspects everything twice a week,” but that “our maintenance crews are getting very limited amounts of time” to do work. Adding to the difficulty: It’s “so cramped and congested” that it’s “labor- and time-intensive to do anything.”

New York and New Jersey can’t keep this up without adding a new rail tunnel and a bigger station. But seven years ago, in one of the worst decisions any modern governor has made, New Jersey Gov. Chris Christie canceled such a link. At the time, Christie cited the project’s ballooning price tag. But a broken commuter artery is costly, too.

Gov. Andrew Cuomo isn’t blameless, either — his billion-dollar plan for Penn Station is a billion-dollar mall, not space for more trains.

Finally, there was President Barack Obama. He was supposed to be the infrastructure president. But we haven’t started building a new tunnel even as the existing one deteriorates. When Christie canceled his own project, Obama never should have let New Jersey keep $176 million in federal funds meant for it.

Trump doesn’t want to add himself to this list. But his recent musings on infrastructure suggest he hasn’t thought about it seriously. Last week, about the Hudson tunnel, he told the New York Times, “I may support” it and “I’m going to look at” it. Nearly three months in, this is an odd stance to take.

Trump also proposed cuts that would defund the tunnel — saying he’ll put together an infrastructure budget later.

That would be OK if Trump was sure of getting an infrastructure budget passed. But he and his staff have also said they might tie infrastructure to some other bill, like health care or tax reform — the latter so complex that any bill could fail after months of negotiation, taking infrastructure with it.

It’s good that Trump has created a commission to figure out how to cut costs and schedules — although, save for a couple of real estate folk, no one knows who’s on it and when it’s going to start and finish its work.

Plus, Trump officials have unrealistic notions of how to pay for projects. Transportation Secretary Elaine Chao told the Times that the president’s trillion-dollar plan won’t cost taxpayers money. “Investors say there is ample capital available, waiting to invest in infrastructure . . . the problem is not money.”

That’s not true. No private investors are going to take the risk of building a tunnel along the densest corridor of the country. Private investors are willing to invest in easy projects, like taking over a toll road. For hard ones, they demand government guarantees.

It’s like the defense industry: It’s hard and expensive to build fighter jets, and no company is going to take the risk all on its own.

Trump is hiking the defense budget, and squeezing those suppliers on costs. He needs to do the same with our train tunnel.

The president should also designate one entity as responsible, rather than continuing the mess where New Jersey Transit, the Port Authority, the MTA and Amtrak all confuse each other. The tax dollars, whether federal or state, all come from the same people.

America can’t be great again if we can’t get to work.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.​ Twitter: @NicoleGelinas