The train, which was travelling at speeds in excess of 80 miles per hour, saw several of its cars spill out onto the Interstate-5 below.
Seventy-seven people were transported to hospitals in Pierce and Thurston counties, the Tacoma-based CHI Franciscan Health healthcare network said.
Pierce County Sheriff's office said "injuries and casualties" have been reported but no numbers are available at this time.
Passenger Chris Karnes told CBS News: "We had just passed the city of DuPont and it seemed like we were going around a curve".
"The next thing we know, we're being slammed into the front of our seats, windows are breaking, we stop, and there's water gushing out of the train".
Train 501 was making its first ever run along the route, which was meant to provide faster service between Tacoma and Olympia.
Jesus Christ's Hometown of Nazareth Just Canceled Christmas Because of Trump
Mayor Ali Salam - a Muslim - said Trump's decision "has taken away the joy of the holiday", the Times of Israel reported . Most of the residents are Arab citizens of Israel - 69 percent are Muslim and almost 31 percent Christian.
In a statement, Amtrak said the train that derailed was Train 501 operating from Seattle to Portland. Add Washington Amtrak Derailment as an interest to stay up to date on the latest Washington Amtrak Derailment news, video, and analysis from ABC News.
Image: The train came off the rail bridge over the I-5 highway.
Photographs posted online from the scene showed multiple cars having derailed from the train.
Amtrak spokeswoman Gay Banks Olson gave the updated numbers at a news conference Monday afternoon.
She said all of the train cars have been searched, and emergency efforts would likely extend through the night and into the morning.
The derailment was Amtrak's second in Washington state this year.
The train had been traveling along a new route connecting the cities of Tacoma and Olympia, which was part of a bigger project meant to reduce travel times.
There were no additional serious injuries to motorists.