In a statement, Amtrak said the train that derailed was Train 501 operating from Seattle to Portland.
Amtrak 501: I'm still figuring that out.
Bova says five vehicles and two semitrailers underneath the train on Interstate 5 were hit when cars came off the tracks from above.
Amtrak president and co-chief executive Richard Anderson said: "On behalf of everyone at Amtrak, we are deeply saddened by all that has happened today".
It wasn't clear how many train cars fell onto the road Monday.
"These are new, upgraded tracks - that's what is so surprising about this", said John Niles with the Coalition for Effective Transportation Alternatives, which joined local elected officials in opposing the project.
"We were coming around the corner to take the bridge over I-5 there right north of Nisqually and we went on the ground". "We were all avoiding one another to try and avoid a collision there".
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A derailed train has slammed into highway traffic during the morning rush hour in Washington State.
Amtrak train derailed roughly 63 kilometres south of Seattle.
In 2015, an Amtrak train going far over the designated speed for a stretch of curves in the track in Philadelphia derailed, killing eight. Amtrak Cascades trains 504 and 509 are canceled, the Amtrak spokesperson said.
The maximum speed along the stretch of track, known as Point Defiance Bypass, is 79mph, according to information about the project posted online by the Washington State Department of Transportation.
Russell Quimby, a consultant who was previously an investigator-in-charge for the National Transportation Safety Board (NTSB), said it appeared to him that the derailment was caused by speed.
The incident marks Amtrak's second derailment in Washington State this year.
But the track involved in today's incident wasn't crumbling. "They are our top priority, and I know first responders are doing everything to ensure everyone has the care they need".