CAYCE, S.C. (AP) — Federal investigators are trying to figure out why a switch was in the wrong position, sending an Amtrak train into a freight train and killing a conductor and an engineer in South Carolina. But they already know what could have prevented the wreck that injured more than 100 passengers: a GPS-based system called "positive train control," which knows the location of all trains and the positions of all switches in an area, and can prevent the kind of human error that puts two trains on the same track. "It could have avoided this accident.