Officials say 72 people were taken to hospitals after most of the Amtrak train's carriages left the track.
A number of those injured are reported to be in a critical condition.
Authorities said all carriages had now been searched, but would not rule out a rise in the number of dead.
Seven
vehicles, two of them lorries, were hit on the I-5 highway below.
Several people were injured in their vehicles but none died.
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 earlier spoke of at least six people killed.
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.
An Amtrak spokeswoman said the train had derailed at 07:33 local time (15:33 GMT).
There were 86 people on board, including 77 passengers and seven Amtrak crew members, as well as a train technician.
Police
say 19 people were taken from the scene uninjured. Of the 72
transferred to hospitals for evaluation, 10 were considered to have
serious injuries.
Barbara LaBoe said the limit on most of the track was
79mph (128km/h) but drivers were supposed to slow dramatically at the
spot where the train derailed.
She said warning signs were in place two miles before the lowered limit. The Associated Press reported that a website mapping train speeds suggested the train may have been travelling at 81mph before the derailment.
The cause of Monday's crash has not been confirmed and investigators have not yet commented on whether speed was a factor.
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.
However
US media outlets pointed out his submitted federal transportation budget
actually proposed cuts to funding to national rail systems.
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