Before passengers feel the first rumbles of an earthquake, BART trains have already slowed or stopped — all thanks to an earthquake early warning system called ShakeAlert.

BART uses ShakeAlert to detect initial, nondamaging waves called P-waves. This allows the system to prepare for the stronger S-waves — which can cause major damage — and slow and stop trains.

“If the messages from the seismic network indicate ground motion above a certain threshold, the BART central computers, which supervise train performance, institute a normal service braking to slow trains down to 26 miles per hour,” said BART director John McPartland in a 2014 statement before the U.S. House of Representatives.

BART was the first adopter of the early earthquake warning system in 2012, which is now expanding across the western United States. The technology can begin slowing trains before riders feel the first shakes, and has gotten even more precise since first implemented, according to Richard Allen, the director of Berkeley Seismology Lab.

Allen was one of the researchers who developed this technology. This past Saturday, he got to witness his research in action when a BART train he was riding came to a stop. The train operator alerted passengers that there was a small earthquake and that all trains had stopped.

“It was very cool to be on the BART train and see this all happening,” Allen said. “The thing we researched for over a decade actually works, and I got to experience it.”

One of the new questions to contend with is how people react to the technology, Allen said. He added that everyone on the train during the earthquake on Saturday was very understanding, and that the train only stopped for a few minutes before it started moving again.

The trains stop automatically, but must be manually restarted again once the situation is deemed safe. They proceed slowly to the next destination to make sure that no tracks are damaged, and then are able to resume normal operations.

There have been no major earthquakes since the new technology was put in place, but BART has successfully stopped for smaller earthquakes. Allen said there have been a handful of false alerts, which have caused BART trains to stop without an earthquake occurring, but have not caused any significant delay.

ShakeAlert technology receives information from seismic stations across Northern California. These sensors are very sensitive and continuously recording, according to Allen. All of the data is processed on campus, where researchers determine the size and location of the earthquake, and early warnings are pushed out. These warnings are released seconds to a minute before the damaging waves begin — it only takes BART about 10 seconds to decelerate from 70 mph to 40 mph, according to BART spokesperson Anna Duckworth.

“This would be crucial to preventing derailments, stopping injuries and keeping the system operational,” Duckworth said. “And the farther the quake is from the BART system, the more lead time there would be for trains to be slowed or stopped.”