Positive train control can prevent deadly crashes, the National Transportation Safety Board has long argued.

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Decades after the National Transportation Safety Board began calling for the crash-prevention tool, Sound Transit showed off its positive train control on Sounder trains Monday.

The satellite-based system, known as PTC, monitors the commuter trains and can stop them if they’re traveling too fast or in danger of a collision. Sound Transit finished the system last fall.

“Even without positive train control, commuter rail is an extraordinarily safe way to travel — far safer than getting on any interstate or major roadway — but with PTC we’re going to make a safe system even safer,” Sound Transit CEO Peter Rogoff said Monday.

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The system was not yet fully operational on the Amtrak Cascades line where a train derailed on a curve near DuPont, Pierce County, in December 2017, killing three people and injuring 62 others. The train was traveling at about 80 mph through a curve where the speed limit was 30 mph.

PTC is now fully operational on all Amtrak Cascades trains, said Washington State Department of Transportation spokeswoman Janet Matkin. Amtrak service has not yet returned to the bypass route where the derailment occurred and won’t until the National Transportation Safety Board issues its final report about the incident, likely sometime this spring, Matkin said.

After a 2008 crash involving a commuter train and freight train killed 25 people in California, Congress mandated PTC on certain train lines by 2015 but later extended the deadline to December 2018. Sound Transit was one of six commuter railroads to meet the deadline, Rogoff said.

Sound Transit began using PTC in 2017, but the system was plagued by technical glitches early on. That year, engineers were able to use the system on 56 percent of trips. Today, it works on “more than 99 percent” of Sounder trips, Rogoff said.

By continuing to “troubleshoot occasional issues” the agency expects to get even closer to 100 percent, he said.

At a cost of $53 million, Sound Transit has installed PTC on 41 Sounder vehicles, the agency said.

About 17,000 riders use Sounder each weekday. PTC will now be in place on all Sounder routes between Everett and Lakewood, the agency said. The funding came from the agency’s general revenues, Rogoff said.

Sound Transit’s Link light-rail trains are already equipped with a similar, smaller-scale system.

PTC uses equipment on the train, along the track and in an operations center to relay GPS information and monitor the train’s speed and location.

The system can track whether the train is complying with speed limits at work zones and other locations along a route. If a train is traveling too fast, the system will warn the engineer and, if the engineer does not respond, automatically stop the train.

The system is meant to prevent trains from colliding with other trains, exceeding speed limits or entering work zones without authorization, said Peter Brown, Sound Transit’s director of systems engineering and integration.

But Brown warned that the system does not detect pedestrians or vehicles such as cars or trucks on the tracks, and so can’t prevent those collisions.