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Sources: Pierce County Sheriff’s Department, Google Earth, Mapzen, OpenStreetMap, federal records and interviews with rail safety experts. Graphic is schematic.

If the engineer doesn’t act, the system takes over, adjusts the speed of the train and, if necessary, brings it to a stop.

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.

Before a trip, data about speed limits, the track, construction zones, locations of other trains and additional information are loaded wirelessly.

In Southern California, Metrolink uses positive train control, or PTC, on all of its routes.

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.

The Amtrak train that careened off an overpass south of Seattle on Monday, spilling three cars onto the highway below and killing at least three people, was traveling at 80 mph in a 30 mph zone, authorities said.

An Amtrak high-speed train derailed from an overpass on Dec. 18, 2017, near Tacoma, Wash., killing multiple people and sending cars flying off a bridge and onto a busy interstate.

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