According to a website which maps the location and speed derived from Amtrak’s train tracker app, the train was going 81.1 miles per hour just before it derailed on an overpass. That part of the track is designed for only 30-mile-an-hour speeds. Passengers say the train never slowed before it flew off the rails.
A head-on crash in 2008 between a passenger and freight train in Los Angeles
pushed Congress to mandate the use of PTC on systems of all major rail lines. While the changes originally required compliance by 2015, there have been hold-ups by Republican controlled congress due to high costs. So this train didn't have one yet.
If the train is moving too fast, the computer on board the train warns the conductor.
In an emergency, the train’s on-board computer can override the conductor & apply the brakes.
Besides slowing and stopping trains that are cruising too fast, Positive Train Control could also be used to prevent collisions between trains (since the system would know where each train is), keep trains from rolling into work zones, and stop trains from cruising through track signals left in the wrong position.