(CNN) The sudden jolt threw Bob Snyder's glasses off his face and knocked his phone out of his hand. On an airplane, it would have felt like normal turbulence, but Snyder was on a passenger train from Seattle to Portland.

The next jolt sent him flying across the train and then back in the direction from which he came.

"That's when I thought this is going to be serious," he told CNN's Erin Burnett. "This is gonna be a big accident."

Snyder was aboard an Amtrak train when it hurtled off an overpass in Washington state Monday morning.

Snyder's car was the first of two cars that ended up on the overpass. Initially, he was worried about the objects flying about inside the train. Then, he looked out the front of the train where there should have been another passenger train and just saw forest.

"That was the first indication of how serious it was," he said.

'Like being inside an exploding bomb'

The train derailed off an overpass near the city of DuPont, sending some of the cars onto a busy Interstate 5.

As Charlie Heebner remembers it, being aboard the train was "like being inside an exploding bomb."

Heebner, 79, and his wife, Beverly, had looked forward to this trip on the Amtrak Cascades 501 train in its inaugural journey from Seattle to Portland, Oregon. The Heebners, longtime train riders, wanted to be on the first run, they told CNN affiliate KOMO

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A jolt and shattered glass interrupted their eagerly awaited trip.

"All of a sudden it was just, crash ... the train went like this," Beverly Heebner, 78, told KOMO, slanting her arms to illustrate.

At least three people were killed, and more than 100 others -- including some highway motorists -- were injured.

Beverly Heebner and her husband, Charlie, survived Monday's train derailment with minor injuries.

"After getting myself upright and my legs unfolded, I saw (my wife's) legs," Charlie Heebner told KOMO. "I reached down and I shook one, and she shook it back. I said, 'Are you all right?' And she said, 'I think so.' "

The couple crawled out of a window of the train, which had been carrying more than 80 people. They told KOMO their harrowing story after they were treated at a hospital for minor injuries and released.

'We were catapulted into the seats'

The train, after running parallel to I-5, went into a curve of the highway. It apparently ran off the track while approaching or crossing the trestle over I-5, with some falling to the road below, landing on vehicles. One rail car dangled precariously over the highway.

The cause of the wreck hasn't been determined. The train was traveling 80 mph in a 30-mph zone, but it's not clear why, National Transportation Safety Board member T. Bella Dinh-Zarr said.

Chris Karnes, a passenger, said he boarded the train between Seattle and Tacoma and took a seat on the third or fourth car.

Karnes said the train was moving at a "pretty good pace," passing cars on the highway while the train was running parallel to the road before it went into the curve over the interstate.

Photos: Amtrak train derails over interstate A train car is lifted away on Tuesday, December 19, the day after an Amtrak passenger train derailed near DuPont, Washington. Several of the train's cars spilled off an overpass and onto Interstate 5. Hide Caption 1 of 14 Photos: Amtrak train derails over interstate Two damaged train cars sit on flatbed trailers after being taken away from the scene on December 19. When it derailed, the train was going 80 mph in a 30 mph zone. Federal investigators said they don't know why yet. Hide Caption 2 of 14 Photos: Amtrak train derails over interstate Men work at the scene of the derailment on December 19. Hide Caption 3 of 14 Photos: Amtrak train derails over interstate Emergency workers respond to the derailment shortly after it happened on Monday, December 18. Hide Caption 4 of 14 Photos: Amtrak train derails over interstate Law enforcement officials work at the scene of the derailment on December 18. The train was making the first trip of a new route that goes from Seattle to Portland, Oregon. Hide Caption 5 of 14 Photos: Amtrak train derails over interstate At least three people were killed and more than 100 were injured, according to DuPont Fire Chief Larry Creekmore. Hide Caption 6 of 14 Photos: Amtrak train derails over interstate Some of the train's wreckage is seen at the crash site on December 18. Hide Caption 7 of 14 Photos: Amtrak train derails over interstate An aerial view of the site on December 18. Hide Caption 8 of 14 Photos: Amtrak train derails over interstate Some motorists were also injured by the fallen train cars. Hide Caption 9 of 14 Photos: Amtrak train derails over interstate The National Transportation Safety Board sent a team to investigate the derailment. Hide Caption 10 of 14 Photos: Amtrak train derails over interstate The train had 86 people aboard, including crew members. Hide Caption 11 of 14 Photos: Amtrak train derails over interstate First responders work at the scene of the crash on December 18. Hide Caption 12 of 14 Photos: Amtrak train derails over interstate All southbound lanes of the interstate were closed due to the derailment. Hide Caption 13 of 14 Photos: Amtrak train derails over interstate The incident took place at 7:40 a.m. near DuPont, which is between Tacoma and Olympia. Hide Caption 14 of 14

"We felt a little bit of a jolt and then at a certain point we could hear crumpling of the train car, and we were catapulted into the seats in front of us," said Karnes, who also is the chairman of the Pierce Transit Community Transportation Advisory Group.

Karnes' car derailed -- only the rear locomotive remained on the tracks -- and passengers kicked out a window and climbed down an embankment to summon help, he said.

Dispatcher: 'What happened?"

Emergency radio transmissions between the train conductor and the dispatcher were frantic and dramatic:

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Amtrak 501: Amtrak 501, emergency, emergency, emergency. We are on the ground! We are on the bridge (unintelligible) on the freeway. We need EMS, ASAP. It looks like they're already starting to show up.

Dispatcher: 501, come in. ... Hey guys, what happened?

Amtrak 501: Uh, we were coming around the corner to take the bridge over I-5 there, right north into Nisqually and we went on the ground.

Dispatcher: ... Is everybody OK?

Amtrak 501: I'm still figuring that out. We got cars everywhere and down onto the highway.

Army officer helps victims off road, train

Army 2nd Lt. Robert McCoy says he hit the brakes of his truck just in time.

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He heard a noise as he was driving on I-5, and glanced ahead to see the train crash through the overpass.

"It hit three vehicles that were in front of me," including a tractor-trailer, McCoy told CNN affiliate KCPQ.

McCoy, assigned to the nearby Joint Base Lewis-McChord, took a tourniquet and a CPR mask from his truck and headed toward the wreck.

People had been ejected from the train onto the pavement, he said. He carried some of them out of the road to safety, and eventually he and another person went on top of the damaged semi and climbed into a suspended rail car.

"There were people yelling. There were people looking for each other, loved ones," he told KCPQ.

Army 2nd Lt. Robert McCoy says he helped ejected train passengers get out of the highway.

He said he helped a woman who was dangling out of the train.

"Her daughter kind of pulled her out backwards and ... I just reached under her, and just kind of got to a point where I could pick her up and just kind of put her down on some form of semi-safe structure," he told KCPQ.

Another motorist, Daniel Konzelman, was driving with his girlfriend on I-5 headed to work when he saw the train "hanging off of the bridge like you've seen in all the pictures," he told CNN on Monday night.

Konzelman, who told The Seattle Times he was an Eagle Scout who'd been trained in first aid, said he got off the freeway and parked his van. Dressed in a suit, he retrieved an emergency head lamp and boots from the back of his van and rushed to the wreck, he told CNN.

He said he tried to give wreck survivors direction, and helped some of them off the train, until first responders arrived.

"Initially, I climbed into the train car that was positioned sideways on the bridge and made sure that everybody in there was stable, and if they could get out, helped them out," he told CNN. "And if they had an injury, (I) made sure that there was people there to tend to them and stay with them.

"And (I) just kind of systematically worked my way through the cars until the first responders got there, and probably was able to help, I would say, 15 people get out before the firefighters or police officers arrived."

'Stuff started flying around'

Passenger Anthony Raimondi had been on plenty of rail rides before -- he worked for 17 years for Amtrak as a ticket agent in Minnesota.

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He was in business class Monday when the car "started to lean."

"Then all of the sudden everything went dark, and stuff started flying around inside the car" before the train came to a stop, he told CNN's "New Day" on Tuesday.

"(We) picked ourselves (up) and shook ourselves off, and one of the passengers pushed out the window," he said.

Raimondi climbed through the window and helped someone else out.

"I feel very, very lucky that I came out with just a bruise here and there," he said.