CAYCE, S.C. (AP) - Federal investigators are trying to figure out why a switch was in the wrong position sending an Amtrak train slamming into a freight train, killing a conductor and an engineer in South Carolina.
But they already know what could have prevented Sunday's wreck - a GPS-based system called "positive train control."
The system knows the location of all trains and the positions of all switches to prevent the kind of human error that can put two trains on the same track.
National Transportation Safety Board Chairman Robert Sumwalt says that system is designed to prevent crashes like the one early Sunday near Cayce that killed the Amtrak train's conductor and engineer and sent 116 of the 145 other people on board the New York-to-Miami train to the hospital.
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