(CNN) Once again, a deadly train crash may have been prevented had a well-known piece of technology been implemented.

Positive train control, or PTC, can automatically slow down and stop a train if it senses the locomotive is going too fast or could get into an accident. The Federal Railroad Administration has called the system the "single-most important rail safety development in more than a century."

Yet the Amtrak train that crashed into a stationary freight train early Sunday morning didn't have PTC, the chairman of the National Transportation Safety Board said.

Had PTC been in place, the catastrophe in South Carolina may have been prevented, chairman Robert Sumwalt said.

The crash, which killed two Amtrak employees, raises familiar questions about what PTC is and why it's still not implemented across the country.

How does PTC work?

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"We consider it an angel on our shoulders out there on the track, and it's our backup and our safety net," said John Hyatt, a former railroad engineer.

PTC was designed to prevent the human errors behind about 40% of train accidents by ensuring that it's being operated in accordance with signals, speed limits or other rules, rail safety experts say. It's an integrated system using tracks and train.

For example, if a train engineer doesn't see a curve coming or a spot where the train is supposed to slow down, signals are sent to the train and "it will begin applying the brakes on the train to get it down to that speed which is required, or stopping the train," Hyatt told CNN.

The NTSB has recommended the use of PTC for decades.

Why wasn't PTC activated?

Sumwalt said he did not know why PTC was not implemented on the train that crashed Sunday in South Carolina.

The disaster stirred memories of a fatal crash in Washington state two months ago. PTC was installed on the segment of tracks where the Amtrak Cascades 501 derailed, but it wasn't operational yet, said Geoff Patrick, spokesman for Sound Transit, which owns the tracks where the derailment occurred.

Photos: Amtrak train derails over interstate A train car is lifted away on Tuesday, December 19, the day after an Amtrak passenger train derailed near DuPont, Washington. Several of the train's cars spilled off an overpass and onto Interstate 5. Hide Caption 1 of 14 Photos: Amtrak train derails over interstate Two damaged train cars sit on flatbed trailers after being taken away from the scene on December 19. When it derailed, the train was going 80 mph in a 30 mph zone. Federal investigators said they don't know why yet. Hide Caption 2 of 14 Photos: Amtrak train derails over interstate Men work at the scene of the derailment on December 19. Hide Caption 3 of 14 Photos: Amtrak train derails over interstate Emergency workers respond to the derailment shortly after it happened on Monday, December 18. Hide Caption 4 of 14 Photos: Amtrak train derails over interstate Law enforcement officials work at the scene of the derailment on December 18. The train was making the first trip of a new route that goes from Seattle to Portland, Oregon. Hide Caption 5 of 14 Photos: Amtrak train derails over interstate At least three people were killed and more than 100 were injured, according to DuPont Fire Chief Larry Creekmore. Hide Caption 6 of 14 Photos: Amtrak train derails over interstate Some of the train's wreckage is seen at the crash site on December 18. Hide Caption 7 of 14 Photos: Amtrak train derails over interstate An aerial view of the site on December 18. Hide Caption 8 of 14 Photos: Amtrak train derails over interstate Some motorists were also injured by the fallen train cars. Hide Caption 9 of 14 Photos: Amtrak train derails over interstate The National Transportation Safety Board sent a team to investigate the derailment. Hide Caption 10 of 14 Photos: Amtrak train derails over interstate The train had 86 people aboard, including crew members. Hide Caption 11 of 14 Photos: Amtrak train derails over interstate First responders work at the scene of the crash on December 18. Hide Caption 12 of 14 Photos: Amtrak train derails over interstate All southbound lanes of the interstate were closed due to the derailment. Hide Caption 13 of 14 Photos: Amtrak train derails over interstate The incident took place at 7:40 a.m. near DuPont, which is between Tacoma and Olympia. Hide Caption 14 of 14

That December derailment killed at least three people and injured more than 100 others on a new route between Seattle and Portland, Oregon. The train was traveling 80 mph in a 30-mph zone, a National Transportation Safety Board member said.

The technology needs to be installed in trains as well the tracks, because a computer system knits both the car and tracks together, Patrick said. The target date for having the PTC up and running for the segment of the track in the December crash had been the second quarter of 2018.

Amtrak has equipped 71% of its locomotives and 67% of its tracks with PTC, according to Federal Railroad Administration data from the third quarter of 2017.

Why isn't PTC mandated throughout US?

Railroad companies have until the end of 2018 and possibly two more years afterward to implement PTC.

Advocacy for PTC has gone on for decades, but the railroad industry has opposed PTC because of its high cost and technological issues.

In response to a 2008 head-on collision that killed 25 people near Los Angeles, Congress passed a law ordering the nation's railroads adopt PTC by December 2015. But as 2015 came to a close, several railroad companies threatened to shut down services unless Congress gave them more time.

So Congress extended the deadline, giving companies until December 31, 2018, with extensions up to 2020 if certain requirements are met.

"The reason why they've been given so many extensions has been money," said Mary Schiavo, a CNN analyst and a former Department of Transportation inspector general. "It is expensive to put it on tracks all over the country."

The Association of American Railroads estimated that as of March 2017, freight railroads had spent $8 billion and passenger railroads $3.5 billion to meet the PTC mandate.

Could PTC have prevented this derailment?

NTSB board member T. Bella Dinh-Zarr has noted that PTC doesn't guarantee 100% safety.

"We should remember that PTC can't prevent every accident," she said. "It does prevent certain types of derailments, overspeed accidents as well as incursions into work zones, for example."

PTC can't prevent accidents if there's trespassing onto the tracks or if a car moves through a railroad crossing, according to the Association of American Railroads.

What other crashes may have been averted?

The NTSB has said PTC technology could have prevented numerous railroad accidents that involved human error.

The agency points to the 2013 Metro-North commuter train derailment in the Bronx, New York, in which four people were killed and dozens injured. The engineer fell asleep and failed to slow the train from more than 82 mph to the maximum authorized speed of 30 mph as it entered a curve, the NTSB said.