The service was supposed to be rated for a maximum speed of 79 miles per hour for the section where it derailed in DuPont, according to passenger Chris Karnes.
But she cautioned that several of the roughly 100 people taken to area hospitals were in critical condition.
Traffic cameras showed a carriage dangling off the bridge onto the highway, blocking the busy road.
The train, which was carrying 78 passengers and five crew, was part of a newly expanded rail service along the route linking Seattle and Portland, Oregon - featuring new locomotives and a new bypass to make the trip quicker.
The National Transportation Safety Board said a "go-team" is departing from Washington, D.C., to investigate the derailment.
A USA official told The Associated Press the death toll is expected to rise and track obstruction is being investigated as a possible cause of the derailment.
'Next thing, we're being slammed into the front of our seats.' People kicked out windows to escape, he said.
A conductor in one of the two engines on the train placed the emergency call. "A lot of them were screaming and moaning, but there was nothing we could do to help them really, so we just stayed with them, we held their hands, we talked to them, we rubbed their backs, tried to get them to just relax and try to be a comfortable as they could", he said.
Emergency crews respond to the scene of a train derailment in Washington state, December 18, 2017.
Image: The train came off the rail bridge over the I-5 highway.
"We will do everything in our power to support our passengers and crew and their families", Anderson said. All told, the project's budget was almost $181 million. The trains were expected to reach speeds of 79 miles (127 kilometers) per hour through the densely populated area with the improved systems and track.
"I knew right away that this was a major accident, that there was probably going to be some pretty, pretty graphic fatalities and injuries, so I prepared myself for the worst going in - and I hoped for the best - and there was both", he said.
US President Donald Trump said the accident underscored the need to invest in infrastructure. Seven trillion dollars spent in the Middle East while our roads, bridges, tunnels, railways (and more) crumble!
At the time, analysts said positive train control technology could have prevented the accident.