What is positive train control, and could it have prevented the Amtrak crash?

DUPONT, WA — The Amtrak passenger train didn’t have a safety technology called the positive train control activated at the time it derailed in Washington state, said Richard Anderson, the rail service’s president and co-CEO.
Positive train control, or PTC, can automatically slow down and stop a train if it senses the locomotive is going too fast or could get into an accident. The Federal Railroad Administration has called the system the “single-most important rail safety