Four cars have gone off the tracks and others are believed to be in danger of sliding in.
Police, fire, medical and other emergency services are on the scene.
The Gig Harbor police tweeted footage of the train lying on its side near the water.
Police are apparently bringing the injured to the marina.
Amtrak's most recent derailments were both at New York's Penn Station, one in late March and one in early April. Both caused huge delays at the major transit hub.
The cause of the accident is not yet known.
Chambers Bay is some 40 miles from Seattle and less than 25 miles from the capital, Olympia.
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