The train derailed and the carriages fell on to the I-5 highway below. Images from the scene show emergency services treating people on the ground. It appears that at least four carriages have left the track; one of them is upside down on the highway.
Several cars on the highway were struck by the derailed carriages, the Pierce County Sheriff’s department says.
Sheriff department spokesman Ed Troyer has told local media that there are fatalities and people still trapped on the train.
Among motorists caught up in the incident there were a number of people injured but no one killed, the department said.
The incident happened near DuPont, which is southwest of Tacoma.
The was the first time an Amtrak train carrying passengers southbound had run on a new section of track running parallel to I-5, called the Point Defiance Bypass.
The crash occurred around 07:30 (15:30 GMT), about 45 minutes into train 501’s journey between Portland and Seattle.
Before the crash, it was travelling at more than 80mph (130km/h), with at least 75 people on board.
The National Transportation Safety Board is investigating.