It was just before rush hour on a Thursday afternoon, and all of the trains between Philadelphia and New Haven, Conn., were at a standstill. But the alert Amtrak sent out did little to calm seething commuters: All it said was that service was suspended because of an “overhead power outage.”

What it did not mention was that a giant circuit breaker — a critical piece of Amtrak’s system for powering trains along the nation’s busiest stretch of passenger railroad tracks — had blown up a few miles from Midtown Manhattan, causing catastrophic damage to an electrical substation and wreaking havoc up and down the Northeast Corridor.

The heat from the explosion seared the paint off the side of the circuit breaker’s metal housing. It melted pins holding insulators on top of it, an Amtrak official said, and severed a critical ground wire. Those breaks sent the high-voltage electric current running haywire, electrifying the whole of the substation’s steel structure. The current then shot through cables back to a control room housed in a ramshackle trailer that, fortunately, was unoccupied.

The disruption on June 20 was just one in a series of delays this summer that exasperated commuters. But more than any of the other failures, the explosion that day showed that much of Amtrak’s vulnerabilities along the Northeast Corridor can be traced back to the system’s astonishing age and long-outdated technology.

Long stretches of that system are unchanged from when the defunct Pennsylvania Railroad first electrified it a century ago, The New York Times has found. And any wholesale modernization effort would come at tremendous cost and take more than a decade.

“It’s staggering, it’s just staggering that we’re still having antique technology controlling our rails,” said John Goglia, a former member of the National Transportation Safety Board whose duties included overseeing railroad investigations from 1995 to 2004. “That’s last century’s technology.”

Michael Bezilla, a historical researcher at Pennsylvania State University who wrote a book on the electrification of trains on the Northeast Corridor, said, “The question is, why hasn’t Amtrak upgraded and why isn’t it using modern technology?”

What was that explosion?

On that June afternoon, as the railroads were gearing up for the homeward rush, the substation suddenly stopped transmitting data to monitors in operations centers in New York and other cities, the official said. Amtrak had lost power on about 10 miles of tracks.

Managers in those hubs were in the dark about what had happened until a worker at a nearby construction site who had heard the blast drove to the substation and called in the first description of the damage, Amtrak officials said.

Workers were dispatched to manually shut down the substation, which feeds electricity through two tunnels under the Hudson River. Meanwhile, two trains were stuck between stations without air-conditioning, one in Queens and another in northern New Jersey. One was towed back to Penn Station. The passengers on the other were transferred to different trains.

That disruption halted Amtrak’s service for several hours, leaving some travelers stranded as trains were halted and canceled as far away as Boston and Harrisburg, Pa. But it also ruined the trip home for thousands of commuters who ride New Jersey Transit trains.

New Jersey Transit shares a segment of Amtrak’s Northeast Corridor, its tunnels and also platforms at Pennsylvania Station in Manhattan. It carries about 170,000 passengers on about 400 trains on the corridor each weekday. Riders were stuck at Penn Station in what Gery Williams, Amtrak’s executive vice president for service delivery and operations, admitted was an “awful experience.”

The two railroads have coexisted uneasily for many years, and 2024 has been no exception.

In late July, several members of New Jersey’s Congressional delegation said in a letter to Pete Buttigieg, the transportation secretary, that “Amtrak disruptions created serious delays for NJ Transit customers no less than 19 times in May and June alone.”

The most recent breakdown occurred on July 31 inside one of the tunnels under the Hudson. After a brief interruption of power to some tracks at Penn Station and in the tunnel, a New Jersey Transit train failed to restart, said Jim Smith, a spokesman for the agency. Passengers were stuck in dark cars without air-conditioning for more than two hours, in the middle of a heat wave, until the train was towed back to Penn Station.

Before the substation explosion, Amtrak officials already were defending against the latest barrage of criticism from New Jersey Transit riders and their elected representatives. They responded by noting that most of the delays this spring had involved New Jersey Transit trains getting tangled in the overhead wires that provide power along the corridor.

Pledging to work more cooperatively, officials of the two railroads investigated the spate of failures. After several weeks, a report on the findings is expected to be released as soon as Tuesday.

But the location of all the trouble was no mystery: Virtually every episode that caused a significant delay for riders happened between two Penn Stations, the infamous one in New York City and the lesser-known one in Newark.

About nine miles long, that segment of tracks serves the most passengers of any in the country — by far. But it is a vestige of the early days of American train travel, when it was owned by the Pennsylvania Railroad, which was known as the Pennsy.

When Amtrak was created in the 1970s, it inherited the Pennsy’s original infrastructure, including the electrical system and the structures that suspend the wires above the tracks to provide power to locomotives.

But the heart of the operation Amtrak inherited relies on electrical standards that are a century old.

Between Washington and New York City, rather than adopting the modern, nearly universal standard of 60 cycles per second, or 60 hertz, Amtrak has patched together an electrical network that runs on the outdated standard of 25 cycles per second, or 25 hertz.

Amtrak maintains equipment that dates to when the Pennsy first electrified the line in the 1920s and ’30s.

The antiquated system has forced Amtrak to cannibalize old substations for parts or have them custom-made, slowing down repairs. The replacement parts for the substation that blew up in June were not expected to arrive until late August.

Madison Feinberg, who has studied Amtrak’s infrastructure as director of electric traction and technical planning for the Effective Transit Alliance, an advocacy group, said: “They’ve upgraded bits and pieces of the system — fundamentally making minor fixes to a design that’s from the ’30s. It’s just effectively playing Whac-a-Mole, which Amtrak’s team is very good at.”

Amtrak officials said there were no immediate plans to update the electrical system.

Bob Comer, who has investigated the railroad industry for nearly 40 years and is president of Forensic & Electronic Research, an independent firm, said railroads in general have resisted a shift to the latest technology for decades, partly because of a culture that is unwilling to spend a dime more than necessary to fix something.

“When you look at that shed exploding,” he said, referring to the breaker housing, “it goes back to the railroad industry being in the dark ages of technology.”

Regarding 25 hertz electricity powering the New York-to-Washington line, Mr. Comer said, “That thing there is like a Model T Ford.”

Kevin Corbett, the chief executive of New Jersey Transit, asserted that Amtrak officials would be the first to admit that “their power system is antiquated.” (New Jersey Transit uses 60 hertz electricity on the rail lines that it owns.)

But Mr. Corbett’s primary concern was the overhead wires that provide power to locomotives and that his agency’s trains keep getting tangled in. Train engines are topped with retractable arms, known as pantographs, that extend to make contact with the wires.

“We need to look at the weakest links,” he said, in a network that he likened to a “rusty old chain.”

The electrical wires hang between steel poles that stand alongside the tracks in a configuration, known as a catenary, that has existed for more than 85 years — also inherited from the Pennsy.

“The structures are from the 1930s, but they’re maintained,” said Mr. Williams, the Amtrak executive vice president.

The system is far from state of the art, though. Amtrak reported last year that none of the overhead wiring and just one-third of the catenary structures between New York and Washington were in a state of good repair. It estimated that remedying the situation would cost about $2.9 billion.

At times, particularly in very hot weather, the wires sag. That increases the chances that a speeding train will break a wire. With just two tracks between New York and Newark, a broken wire brings traffic on the rails to a halt, like it did several times this year.

Over a three-day period in late May, there were four incidents involving overhead wires that caused significant delays, Amtrak reported. In one case, a signal wire fell onto a 12,000-volt power cable, blowing out fuses and circuit breakers along a five-mile stretch of the corridor. It took 12 hours to restore full service.

When will everything be fixed? Years. Many years.

For the first time in its history, Amtrak is awash in federal funding for its national network. It was a big beneficiary of the $1.2 trillion infrastructure spending package that Congress passed in 2021. Amtrak officials have concentrated on major projects like Gateway, which would add a pair of one-track rail tunnels under the Hudson between New York City and New Jersey.

The first phase of the sprawling Gateway project is the replacement of the 114-year-old Portal Bridge, just west of the tunnels. Amtrak officials frequently point out that when that new bridge is completed, it will come with about three miles of new track and catenary. The planned replacement of the Sawtooth Bridges in Kearny, N.J., would produce another new segment, they say.

In June, Amtrak’s chief executive, Stephen Gardner, told a Congressional committee that his agency would need an additional $100 billion over the next 13 years to complete all of its planned repairs and improvements to the Northeast Corridor.

Some elected and appointed officials in New Jersey are pressing Amtrak to abandon the piecemeal approach to upgrading the corridor and make wholesale improvements to the parts that are causing the worst delays.

In their letter to Mr. Buttigieg, the New Jersey representatives said that $6 billion that Congress had appropriated for maintenance of the Northeast Corridor “should be used to address the problem right now.”

The awarding of federal money for railroad repairs and upgrades starts in Congress, which allocates funds to the Transportation Department. That money is then controlled within the department by the Federal Railroad Administration.

Amtrak has applied for as much as $300 million in grants from the federal Transportation Department to make improvements along the corridor, including the replacement of an electrical substation in Kearny that was flooded by Hurricane Sandy 12 years ago. But that substation would operate on the same outmoded standard as the one that blew up in June.

The key to a total overhaul of the corridor in the New York metropolitan area is the completion of the $16.1 billion Gateway tunnel project, Amtrak officials say. Only then will the system have the additional tracks that would allow for the replacement of all of the antiquated infrastructure the region relies on.

The most optimistic projection for when that might happen is 2035.