A number of people died and many more were injured on Board the train, which hit vehicles on the I-5 highway below.
Images from the scene showed that all the carriages of the high-speed train had left the track.
It was Amtrak's first passenger service to run on a new, shorter route.
Train 501 had left Seattle, heading south for Portland, at 06:00 local time (14:00 GMT). The derailment happened on a section of track, south of Tacoma, previously only used for freight trains.
Authorities quoted by Associated Press spoke of at least six people killed, although the number was expected to rise. Another 77 people have been taken to hospital, reports say.
However, state police said they were giving out no information on the number of casualties.
They did not know if anyone remained trapped on board, Trooper Brooke Bova said.
"We still need to stabilise those cars so we can do a secondary search," she added.
Brooke Bova refused to say whether anyone had been hurt on the road itself.
Seven vehicles, two of them lorries, were hit on the interstate highway below.
One passenger carriage could be seen dangling from the bridge, while others were strewn across the road and the wooded area next to the track.
A photograph released by Washington State Patrol showed one carriage upside down on the road, with wreckage all around it.
Pierce County Sheriff's department spokesman Ed Troyer said there were "multiple" fatalities on the train but no motorists had been killed.
An Amtrak spokeswoman said the train had derailed at 0733 local time (1533 GMT).
There were 77 customers and seven Amtrak crew members on board, she said.
A recording of the train's emergency call to railway dispatchers was released to US media.
"Emergency! We are on the ground!" a man, possibly the conductor, radios in.
In a second radio call, another crew member reports that only the rear unit remains on the rails.
"All other cars appear to be on the ground in quite a mess," he says.
The train's engineer has a head injury, he tells dispatchers.
President Donald Trump's first reaction to the derailment was to tweet that it showed the need for his forthcoming infrastructure plan.
Passenger Chris Karnes, on board the train, said his carriage had careered down an embankment then come to a stop: "We could hear and feel the cars crumpling and breaking apart, and water came out from the ceiling."
"In order to get out... we had to kick out the emergency window," he added.
Governor Jay Inslee said he was praying for the many injured.
Officials set up a family reunification centre at DuPont city hall and asked people not to come to the scene of the crash.
A local news reporter who was on the train, but got off at a stop just before the crash, said many of those on board were railway enthusiasts, keen to experience the first high-speed train on the new route.
Every passenger was given a commemorative lanyard and badge to mark their journey, he said.
The National Transportation Safety Board is investigating. An Amtrak spokeswoman would not comment on how fast the train was going as it approached the bend, or on speculation that there might have been something on the tracks.

What do we know about the train?

• The train involved was operated by Amtrak and Monday's service was the first 0600 departure under the new timetable and on the new Point Defiance Bypass route
• The train consisted of 14 cars, two of which were locomotives
• It was being led by a Siemens Charger locomotive and was trailing a P52 unit which was not under power, Amtrak said
• There were 12 Talgo carriages, each of which could take up to 36 passengers, but only 77 customers were on board