Robert S. Sumwalt, chairman of the National Transportation Safety Board, has been there before and on Thursday he virtually begged a congressional subcommittee to end the habit of holding hearings without more meaningful results.

Testifying before the House Subcommittee on Railroads, Pipelines, and Hazardous Materials, he rattled off a list of fatal train wrecks that would not have happened had a device known positive train control (PTC) been put in place by the railroads.

“Let me spell it out,” Sumwalt said, “for each day we go without PTC, we are at risk for another Chatsworth, another Bronx, another Amtrak in Philadelphia, another DuPont or another Cayce, S.C.”

Those five train wrecks alone killed 42 people and injured hundreds. As Sumwalt knew all too well, they are among the many human-error train crashes that could have been avoided with PTC. He said his agency has investigated 22 train wrecks since the crash 10 years ago in Chatsworth, a Los Angeles neighborhood, that PTC would have prevented.

“These accidents have resulted in 29 deaths, more than 500 injuries and over $190 million in property damage,” Sumwalt said.

Thursday’s hearing was deja vu for Sumwalt. He’d been before the same subcommittee in February. Then he said, “The NTSB is extremely concerned about any further delay to this lifesaving technology.” In 2016, months after rushing up to an Amtrak wreck in Philadelphia — eight dead, 185 passengers taken to the hospital — he lamented, “Every day that PTC is not in place, we run the risk of another Amtrak crash. Is it going to take another five years or another three years for it to be implemented? If that’s the case, that’s unacceptable.”

Sumwalt on Thursday got an answer to that question he asked two years ago: Some railroads have PTC in place, 65 percent of them are either operational or in testing, and 35 percent of 41 railroads will fall short. It’s unclear on exactly how many railroads PTC is operational — and it’s important to remember that several rail lines can operate on the same track — but the Federal Railroad Administration anticipates most railroads that qualify will need extensions from a Dec. 31, deadline to make their PTC operations functional.

“While railroads are making progress, FRA [believes] that most railroads will need to request an [extension beyond December] to complete testing, obtain systems certification and complete interoperability requirements,” said Ronald L. Batory, administrator of the Federal Railroad Administration. Interoperability is jargon for the ability of trains from other rail lines to use PTC signals from towers operated by their host railroad.

Despite Batory’s contention, Edward R. Hamberger, president of the freight railroad lobby Association of American Railroads, told the committee that 80 percent of railroads covered by the rule would have PTC in operation by year’s end.

“While some class 1 railroads expect to be fully operational by the end of the year, all class 1 railroads will be fully implemented no time later than sometime in 2020,” Hamberger said.

Hamberger added, “Rigorous and repeated testing is the only way to make sure it works."

Susan Fleming of the Government Accountability Office told the committee that as of June 30, many railroads remained in the early stages of PTC implementation and that many had begun testing their systems.

“Moving for field testing . . . has taken an average of two years to complete,” Fleming said. “About a quarter of the railroads told us they had encountered software bugs.”

The frustration of the House subcommittee has surfaced with regularity at the multitude of hearings on PTC.

“I have zero sympathy for the railroads that have done nothing,” said Rep. Michael E. Capuano (D-Mass.) “It’s only a matter of time before the next accident happens and someone dies. How much is a life worth?”

The NTSB is an independent body created by Congress to investigate transportation crashes. Unlike Congress or federal agencies, it has no hand in shaping policy. All it can do is make recommendations to Capitol Hill or federal agencies. It has recommended implementation of PTC since 1990.

“Is that acceptable?” Sumwalt said of the PTC delay at Thursday’s hearing. “It’s certainly not acceptable to the NTSB.”

Since a 1988 train wreck just south of Philadelphia when an Amtrak train collided with a piece of railroad maintenance equipment, injuring 34 people, the NTSB says that PTC could have prevented 141 deaths and 2,426 injuries.

A pivotal moment for PTC came 10 years and a day before Thursday’s hearing. A Metrolink commuter train whose engineer was busy swapping text messages with a friend missed a signal in Los Angeles and allowed his train into the path of an oncoming Union Pacific freight train. The trains, weighing 1,800 tons between them, slammed into each other head-on at 85 miles an hour. The Metrolink train was split open as if with a can opener, killing 25 people and injuring 102.

Though Congress already had been considering a requirement for some form of PTC, the wreck in Los Angeles catapulted it into action. It passed legislation — signed into law by President George W. Bush a month later — that mandated PTC be installed on 58,000 of the nation’s 134,000 miles of railroad track, including those most heavily used by passenger rail lines.

The railroad industry, however, bridled at what it said was an investment of almost $15 billion to install computers in locomotives, trackside towers to communicate with those computers and to train workers to use them. After Republicans became the majority party in the Senate in 2015, railroads that had contributed about $58 million to House and Senate campaigns, were able to prevail on Congress to extend the PTC deadline by three years to 2018, and to allow exemptions until 2020 for railroads that were making substantial progress toward implementation.

The NTSB has said that since the crash of Sept. 12, 2008, in the Los Angeles neighborhood of Chatsworth, among the fatal train wrecks that PTC could have prevented were these:

•Red Oak, Iowa — Two crew members died when their coal train hit a maintenance train in 2011. Contributing factor? The NTSB report says: the “absence of a positive train control system.”

•Mineral Springs, N.C. — Hours before dawn, two crew members died when their speeding train slammed into the back of a train waiting for a signal to change in 2011. NTSB conclusion: “Had a positive train control system been installed on this track, it could have prevented the collision.”

•Goodwell, Okla. — In 2012, three crew died in a head-on train crash when an engineer missed a signal. The NTSB cited the “lack of positive train control, which would have stopped the train and prevented the collision regardless of the crew’s inaction.”

•Bronx — In 2013, an engineer dozed off on an early-morning commuter train, allowing it to speed into a turn at close to three times the posted limit. Four passengers were killed. Had PTC been in place, the NTSB said, “that would have automatically applied the brakes to enforce the speed restriction.”

•Hoxie, Ark. — In 2014, dozing crew members missed a signal at 2:28 a.m. and crashed head-on into another train, killing two of them. The NTSB said, “A functioning positive train control system would have prevented this accident.”

•Philadelphia — An engineer’s “loss of situational awareness” caused an Amtrak train to barrel into a curve at more than twice the permissible speed in 2015. Eight passengers were killed. NTSB said: “The accident could have been avoided if positive train control . . . had been in place.”

The NTSB has not determined the probable cause of crashes in Washington state in December and South Carolina in February that killed five people and injured 154 combined. But the agency’s preliminary reports suggest PTC could prevented both crashes.