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Down. if he runs past a red signal, the it will automatically stop him. it's based on technology that's been around for decades. now, that would only occur if this were a situation, again, like the may 2015 philadelphia amtrak crash where the train operator was going around a curve much too fast or the one back in 2013. it would slow it down. but if there was a rail break, like the rail broke under the train or an axle broke, positive train control isn't going to do anything. >> i also want to bring in on the phone former ntsb investigator in charge and a railroad accident investigation expert...

And that required some -- as you can see in some of the pictures, the traffic situation was horrible. they say the emergency rooms are fully up and running and taking those patients in. let me go back to you, gus, because nbc has learned that these tracks have what is called positive train control. explain for folks who don't know this stuff what is positive train control and what could that tell us about this? >> positive train control was something that the federal government mandated after the 2008 metro link crash in california. basically as far as accidents go, it says that if -- there's ...

>> contributing to the accident was the absence of positive train control or ptc. all of these deaths and injuries were preventable. it would have prevented this derailment by automatically applying the brakes even when the engineer did not. the ntsb has called for the implementation of ptc. thankfully congress has mandated the use of this technology by the end of next year. thank you, congress. >> thank you, congress. but no rush. we are really good at investigating these kinds of disasters when they happen we find out why they happen and we then find out what would have presaented them from ...

Would not have died. >> contributing to the accident was the absence of positive train control or ptc. all of these deaths and injuries were preventable. it would have prevented this derailment by automatically applying the brakes even when the engineer did not. the ntsb has called for the implementation of ptc. thankfully congress has mandated the use of this technology by the end of next year. thank you, congress. >> thank you, congress. but no rush. we are really good at investigating these kinds of disasters when they happen, we find out why they happen, and we then find out what would hav...