Positive train control — the technology that uses sophisticated electronics to monitor trains and take control of them if necessary to prevent collisions, derailments and other crashes — wasn’t in use on the stretch of track where the derailment occurred, Amtrak President Richard Anderson said.
In Southern California,
How PTC works
Before a trip, data about speed limits, the track, construction zones, locations of other trains and additional information are loaded wirelessly.
En route, the train’s position and speed are tracked via GPS. Signal status, broken rails and curves are provided from wayside devices. Using such data, the system tells the engineer when to adjust the speed of the train.
If the engineer doesn’t act, the system takes over, adjusts the speed of the train and, if necessary, brings it to a stop.
Sources: Pierce County Sheriff’s Department, Google Earth, Mapzen, OpenStreetMap, federal records and interviews with rail safety experts. Graphic is schematic.