If the engineer doesn't push the button to acknowledge the alert, the train will stop within eight seconds.
"What we've been trying to build here is a system where humans back up machines, machines backup humans, so that if there is a failure, there is another system in place designed to protect passengers and the crew," said Richard Katz of Metrolink.
In 2012, Metrolink trains will be equipped with positive train control, a GPS technology that will be able to stop a train immediately.
However, Metrolink said it couldn't wait three years for the safety enhancement in light of last year's crash in Chatsworth that killed 25 people. Had the existing ATS technology been on board, the accident could have had a different outcome.
"It may not have stopped them in time, but it would have slowed them down, and that would have made a difference, and I would bet that lives would have been saved," said Katz.
Once positive train control is installed on Metrolink trains, the automatic train stops will become obsolete, but Metrolink says it's money well spent in the meantime.
Passengers agree that their safety can't wait.
"That's wonderful; I don't see why they didn't have it long ago," said commuter Billy Joe Wright.
"I feel safe on the train. I know that they're trying, they're going to put stuff in and they're spending the money, so I feel like I'm safe," said another commuter Victoria Fuentes.
ATS is in place on what Metrolink calls "priority locations," where the trains approach a curve or change speed.