In the wake of the Chatsworth collision, Automatic Train Stop is being installed elsewhere on Metrolink (it had already been in place on portions of the former ATSF line to San Diego). ATS is a reactive system; if a train improperly passes a signal it will apply the brakes. PTC is a predictive system; among other things, if a train does not begin braking in an adequate distance ahead of the system will intervene. This is not a simple concept when you consider the system no only needs to "know" the location of the train but also the location of the limits of the train's movement authority, and the location of speed permanent and temporary speed restrictions. It also needs to know the train's braking capability (which can vary significantly depending on type of train, distribution of loads / empties, etc.)