Ten years after a Metrolink commuter train and a Union Pacific freight train collided head-on at 84 miles per hour, killing 25 people and injuring 135 in Los Angeles, the safety system that could have prevented the crash still isn’t working on at least a third of the nation’s rail network.

“Had a fully implemented positive train control system been in place . . . the collision would not have occurred,” the National Transportation Safety Board (NTSB) said after a comprehensive investigation of the collision that took place Sept. 12, 2008, in the Chatsworth neighborhood.

Congress soon thereafter passed legislation mandating that positive train control, commonly known as PTC, be installed on nearly 58,000 miles of railroad — including the most heavily traveled passenger lines in the nation’s rail network.

When federal regulators announced last month that PTC was 65 percent in place, the news came with this caveat: “PTC is in operation or advanced testing.” The 65 percent includes significant track systems where it is not fully operational, despite a Dec. 31 deadline for completing the process.

According to the NTSB, in the 10 years since the Chatsworth crash, at least 21 deaths and 364 injuries have occurred that could have been prevented had PTC been in use.

The NTSB has yet to determine the probable cause of crashes in Washington s tate in December and South Carolina in February that killed five people and injured 154 combined. But the agency’s preliminary reports suggest that PTC could prevented both crashes.

NTSB records show that PTC could have prevented 141 deaths and 2,426 injuries that have occurred since 1988, when a wreck between an Amtrak train and a piece of railroad maintenance equipment injured 34 people in Chester, Pa.

PTC is designed to eliminate human error from train travel. If a train is going too fast, it automatically applies the brakes to comply with set speed limits. If a track switch is improperly positioned, it prevents the train from going down the wrong track. To prevent rear-end or head-on collisions, it keeps two trains off the same track.

“Every day that PTC is not in place, we run the risk of another Amtrak crash,” NTSB board member Robert L. Sumwalt said in a moment of exasperation in January 2016.

“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,” said Sumwalt, who now chairs the panel.

His forecast was correct: Despite another implementation deadline this year, most freight and passenger railroads plan to seek an exemption that will delay flipping on the PTC switch until December 2020 at the latest.

Though the NTSB has recommended PTC installation since 1990, and Capitol Hill already was in motion after several deadly crashes — including a 2005 South Carolina wreck that leaked chlorine gas, killing nine, sending 556 to the hospital and requiring thousands of nearby residents to evacuate for days — the death of 25 people in Chatsworth finally got Congress over the hump to pass legislation.

“For nearly 50 years, the NTSB has issued one recommendation after another for the [Federal Railroad Administration (FRA)] to require the railroads to implement some form of positive train control,” said Rep. Peter A. DeFazio (Ore.), the ranking Democrat on the House Transportation Committee. “Those recommendations fell on deaf ears and it took the Chatsworth crash for Congress to finally take action to force PTC implementation. We can’t afford any further delay in adoption of this lifesaving technology.”

Signed into law by President George W. Bush a month after the Chatsworth crash, the legislation mandated that PTC be in place on nearly 58,000 miles of railroad by the end of 2015.

When Republicans came to dominate both houses of Congress in January 2015, railroads’ powerful lobby saw an opportunity to seek relief from the deadline, which it found onerous.

Members of Congress have received nearly $60 million from railroad lobbyists since 1990, according to data compiled by the Center for Responsive Politics.

The railroad industry argued successfully in 2015 that the mandate required a rush to invest in technology they said was too complicated and not fully developed. They argued that the nearly $15 billion required to educate rail workers and install onboard computers in engines and communication towers along more than 40 percent of the nation’s 134,000 miles of freight and commuter lines was prohibitive.

Congress relented, extending the deadline to the end of this year, but it added a caveat. If a railroad had installed the necessary PTC hardware, trained rail workers, acquired radio spectrum and had the system ready on 50 percent of the required lines, they could seek an exemption until the end of 2020 to turn on PTC.

The FRA issued a progress report last month that said that 15 of 40 railroads had installed all the required equipment and that 12 others were close to that achievement.

Notable, however, is that the railroad that has been the poster child for PTC progress — the massive freight rail line BNSF — was the first railroad to seek an extension beyond the Dec. 31 deadline. Although BNSF is fully equipped, has every bit of its tracks covered, and all its employees trained, it has a software issue that is keeping its system from being fully operational. Twenty-four railroads operate on BNSF tracks, and only six of them are PTC-capable.

The kinds of extensions railroads are seeking are not what Congress intended.

“An extension was intended for testing, where a few bugs had to be worked out to ensure the highest level of safety, prevent PTC failures, or the FRA needed a little time to certify each railroad’s safety plan,” said Jennifer Homendy, a former railroad expert on the House Transportation Committee, who was sworn in as one of five NTSB members last month.

“The railroads should not be at the stage where hardware is still being installed and employees still haven’t been trained,” Homendy said. “They should be well beyond that. Yet the latest reports submitted to FRA show that while some railroads have made tremendous progress, others have done little to nothing toward implementation.”

The results reported by FRA last month show a wide disparity in progress by both the freight and passenger rail lines. Most of the big freight railroads have made progress, but they appear likely to seek extensions to complete the job next year or by the end of 2020.

Commuter rail lines run the gamut, from Philadelphia’s SEPTA (100 percent equipped and trained) to a New Mexico line that FRA reports has done nothing at all.

Ed Hamberger, president of the Association of American Railroads, a coalition that lobbies on behalf of freight railroads, said it is “on track to meet the deadlines established by Congress” and “will have PTC fully implemented and operational on or before the 2020 deadline.”

One success story: Metrolink, the commuter rail line involved in the Chatsworth crash. It has had PTC in full operation since June 2015.

“Nothing focuses your attention like an incident such as that that occurred in 2008 for Metrolink,” spokesman Paul Gonzales said. “PTC is on all the time. There are no near misses, there are no close calls. Any condition that should arise that would lead to a dangerous situation gets handled by the PTC system.”

The Metrolink engineer that day 10 years ago had sneaked a prohibited cellphone onto the train and was busy swapping text messages with a friend, according to the investigation. He blew through a stop signal. The collision with the freight train split his train as if with a can opener. He and 24 of his passengers died.

The worst train wreck in U.S. history took place in Nashville 100 years go, when 101 people were killed and 171 were injured in a head-on crash at more than 100 mph on “Dutchman’s Curve” west of downtown. Investigators concluded that errors by one engineer and by people in a control tower were to blame.

That is precisely the type of human error that PTC would prevent.