More than five years after a collision killed 25 people and spurred new railway regulations, Southern California's Metrolink has become the first commuter rail system in the country to implement a new automatic safety system in revenue service.

The system, dubbed positive train control, uses GPS to monitor train movement, alert conductors and dispatchers about potential collisions, and even take control of the train to prevent a crash. It's especially useful if crews are working on the track, switches are left in the wrong position, or engineers disregard instructions from dispatchers.

But most importantly for Metrolink, it fulfills the requirements of the Rail Safety Improvement Act of 2008, which requires rail operators to install positive train control by December 31, 2015. Passage of the requirement became a priority shortly after a Metrolink train collided with a Union Pacific freight train in Chatsworth, California, in 2008. It was reported that the train's engineer was texting immediately before the crash, and the Metrolink train ran a red light.

It may seem like common sense to install electronic fail-safes that prevent rail accidents, but implementing the technology is extremely complex. Unlike the collision-prevention systems found on modern automobiles, positive train control requires extensive electronic communication with dispatch and coordination with other rail carriers to work properly. That's a major concern for rail operators.

It also is expensive. The PTC system is currently operational on selected trains running between Los Angeles and Riverside. Installing the technology on all 52 locomotives and 476 signals throughout the system, which covers a wide swath of Southern California, will cost nearly $211 million. A full scale rollout of positive train control by the entire rail industry could cost more than $10 billion.

Analysts and executives have questioned the U.S. rail network's ability to meet the PTC mandate, and not just because it'll cost them money. Getting the nation's railways to communicate with each other over a patchwork of wired and wireless networks is a Herculean task. Even if all the passenger and freight railways agree on standards and work with suppliers to develop proprietary communications equipment, the sheer number of trains means that there may not be enough available bandwidth in large cities for trains to talk to one another. Metrolink goes so far as acknowledge how much of a challenge it will be to acquire that much bandwidth.

It's also important to note that PTC can't override the laws of physics. If a truck gets stuck on tracks at a crossing and a train doesn't have time to stop, a crash will still happen.

Metrolink's implementation of PTC is a major milestone for a technology that many thought would never see the light of day. But it's useless until other agencies get on board—and it's highly unlikely that will happen by 2015.

Photo, Video: Metrolink