Transcript for Amtrak derailment survivors, eyewitnesses, describe scary scene
Oh my god! Reporter: It's every commuter's worst nightmare. Amtrak 501 north, over. Amtrak 501, emergency, emergency, emergency. We are on the ground. We are the bridge on I-5, derailment -- Reporter: A train traveling Seattle to Portland, derailing while crossing an overpass. They're requesting all possible units responding on the freeway. Reporter: At least three people dead, on board approximately 84 passengers and crew members, 13 of 14 train cars jumped the tracks. Some spilling over onto busy interstate 5 below. Oh my god that train is derailed over the freeway. Reporter: Some train cars suspended in the air. Others crushing vehicles below. Within minutes interstate 5 awash with red and blue lights. That is crazy. People were in shock. And a lot of them, they just went and they sat down with their faces in the palm of their hands like, what's happening? Reporter: Daniel and his girlfriend Alicia were driving by and stopped to help. I went down underneath the freeway and started working my way through those trains. One of the trains had tipped upside down and spill the passengers out. Then the train came down on top of them. One of the gentlemen had been thrown from the train. He was one of the fatalities. Reporter: 100 people, including passengers, crew, and motorists below, rushed to nearby hospitals. There are at least three fatalities. Amtrak's cascades train number 501 was on its inaugural passenger run, traveling down a rail bypass formerly used for freight but was modified to carry passenger trains. The train departed Seattle at 6:00 A.M., scheduled to reach Portland at 9:20. 7:30 A.M., passenger Chris Carnes, chair of the pierce county transit advisory board," wow this train is fast, we are passing up traffic on I-5." Moments later around 7:34 A.M., a locomotive and 12 cars careening off both sides of that very overpass. We were going between 70 and 80 miles an hour. And we heard a creaking. A little bit of a creaking sound. Then the train started to wobble. Reporter: Just below that busy southbound traffic. And then we were catapulted at the seats in front of us. The next thing we knew, our car had crumpled, and we were down an embankment. I looked around, there was windows and glass and a particular smell that I can't really describe. Reporter: The conductor radioing for help. Hey, guys, what happened? Coming around the corner to take the bridge over I-5 there, right north into squally, we went on the ground. Is everybody okay? I'm still figuring that out, - we got cars everywhere and down onto the highway. As soon as I know about exactly where all my train, I'll let you know. The first car was on the ground, the second car was the passenger, was down. The third was dangling. The fourth and fifth were up on the track. About 20 ems started flooding toward the area. A lot of people in the northbound lane was going over to help as well. Reporter: The cause of the derailment is still being investigated. Everyone's familiar with black boxes on airplanes. But trains also have similar recorders. They're capturing parameters such as speed, throttle position, braking, and even whether or not they're blowing the whistle or the horn on the train. So these devices can be really helpful to investigators to really corroborate with what they're hearing from witnesses. Reporter: But the owner of the track where the train derailed confirmed to ABC news that the speed limit on that portion of the track is 30 miles per hour. Officials have not yet confirmed how fast this train was traveling. Earlier this month, the mayor of Lakewood, Washington, expressed concerns about safety. Come back when there is an accident and try to justify not putting in those safety enhancements. Or you can go back now and advocate for the money to do it. Because this budget project was never needed. When we got to the scene it was obvious there were fatalities and injuries. Multiple cars and trucks were struck by train cars that left the tracks and went down onto the road. The people in all the vehicles, even though when you see the pictures, it's rate horrific, at this point nobody in any of the vehicles is a fatal. The fatales are all contained to the train. Reporter: A medical tent was set up nearby to help triage the numerous victims. Nearby hospitals asking for blood donations. We also have crew members who have survived. And so interviewing them to understand what was going on immediately prior to the derailment is going to be very important for the investigators and they'll want to do that as soon as possible. Reporter: Today's Denmarks the second derailment this year in the same county. The first back in July when a passenger train carrying 267 people skidded off the tracks. They slam on the brakes and we're all of a sudden tilted. Reporter: No casualties, only a handful of minor injuries. However, an investigation later showed the train's engineer was driving too fast, triggering a derailment switch on the tracks prior to reaching a draw bridge. Notify amtrak to shut down the entire northeast corridor. We have a major event here. Reporter: Speed also a factor in 2015 when an amtrak passenger train traveling twice the speed limit sped off the tracks outside of Philadelphia. Crawl forward, sir! Reporter: This is all tafs LE that was left of the train, carrying 238. The derailment killing eight, hundreds injured. A report would later note a culture of safety lapses at amtrak was responsible for the collision, adding the accident could have been prevented if the tracks had been equipped with a system that enforces speed restrictions, known as positive train control, PTC. Positive train control was mandated by law, but amtrak has been very slow to implement it. What the U.S. Is putting in right now is a rudimentary auto pilot that in aviation we would find in the 1940s. Reporter: Sound transit, the owner of the track, says PTC was installed here but in a phone conference amtrak's CEO Richard Anderson told reporters PTC was not activated. He did not want to speculate on the speed of the train or scenarios that could have caused the derailment. There's still too much room for human error. There's still too much old technology. There's old track, old trains. We're dealing with a train system that's decades behind the rest of the world. This is a tragic event. On behalf of everyone at amtrak I want to express that we're deeply saddened by what happened here today. Reporter: Back in Washington, debris and twisted wreckage still block two major transportation veins. There's a thorough investigation under way to determine what happened. Reporter: Tonight investigators trying to answer that lingering question. What caused the train to derail and claim one too many lives? I'm Clayton Sandell for "Nightline" in Dupont, Washington.
This transcript has been automatically generated and may not be 100% accurate.