Tuesday night's derailment of Amtrak Northeast Regional 188 has claimed at least seven lives, injuring dozens more. The National Transportation Safety Board (NTSB) has now taken over the investigation — which could last weeks, or months — but The Wall Street Journal is already reporting that the train is believed to have been traveling over 100 miles per hour near a curve designed for a speed limit of just 50.

How does that happen?

Human error is all too real in every form of transportation — operator fatigue or carelessness can certainly cause a train to go too fast, and it sometimes does with tragic results — but the technology to stop trains from going too fast already exists. It's called Positive Train Control (PTC), and it will be federally mandated on major railways nationwide by the end of this year — a controversial requirement that many in the rail industry have attempted to delay. PTC is designed specifically to avoid collisions and overspeed derailments through a system of transponders and GPS receivers that allow computers to take over when an operator isn't handling the locomotive correctly.

Even more tragically, though, Amtrak has already been trying to implement its own version of PTC, called ACSES, throughout the Northeast Corridor — the segment of track that carries thousands of commuters between Washington DC, New York City, Boston, and points in between. According to a report in Amtrak's employee newsletter from earlier this year, the company's implementation was operational from Boston to New Haven; New Brunswick to Trenton, New Jersey; and Perryville, Maryland to Wilmington, Delaware. The entire Northeast Corridor has been promised PTC for several years, but as of this year's report, Amtrak is still trying to finish installation. It's unclear whether PTC was operational on the specific section where 188 derailed, but there is no indication that it was — and a Reuters report quotes unnamed officials that PTC was not installed on that specific train.

"I do not know right now whether or not Amtrak's PTC system called ACSES has been installed and is operational on all four tracks on this line north of Philly. If it was installed and operational it should have prevented the derailment," says Steve Ditmeyer, an adjunct railway professor at Michigan State University.

This isn't the first time PTC — or the lack thereof — has been cited as a contributing factor in an accident. It's widely believed that the 2013 MTA Metro-North derailment, where a train came off the tracks going 82 miles per hour in a 30 mph zone, could have been prevented had PTC been able to take over.

"This upgrade is going to help reduce train accidents," reads an unfortunately prescient quote from an Amtrak radio maintainer in Amtrak's newsletter.

Amtrak did not respond to a request for comment in time for this report.

Update May 13th, 3:47PM: Added a reference to a new Reuters report that PTC was not installed on the train involved in the incident.