As they begin to probe the circumstances of the fatal wreck in South Carolina, officials have already settled on one thing: that the crash between a passenger train and a freight car could have been prevented with technology already installed in parts of the U.S. It's been a decade since Congress approved a law mandating the GPS-based system called "positive train control," which is designed to prevent two trains from traveling on the same track at the same time. That's what happened early Sunday in Cayce, South Carolina, when a locked switch sent a New York-to-Miami Amtrak passenger train onto a side track where an empty CSX Corp. freight train was parked.