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.