By Jason Hanna, CNN

(CNN) -- The way Charlie Heebner remembers it, being aboard the passenger train that hurtled off an overpass in Washington state Monday morning was "like being inside an exploding bomb."

Heebner, 79, and his wife, Beverly, had looked forward to this trip aboard 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.

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.

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

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

"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.

"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:

Dispatcher: 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.

Witness helps people off the train

Daniel Konzelman was driving with his girlfriend on I-5 headed to work when he saw the train running parallel to him, "going faster than us."

Ahead, the train headed into the curve over the highway.

"We saw the train that had just gone by us about 45 seconds before, hanging off of the bridge like you've seen in all the pictures," Konzelman told CNN on Monday night.

"I was like, 'Is that like the train that just went by us?' And it then like, sort of settled in, 'Whoa, this is major. Major train wreck.'"

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.

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.