The Amtrak 501 train – the first high-speed commuter service to use a new track from Seattle to Portland – was moving at 80mph in a 30mph zone, federal officials said.
A data recorder recovered from the train’s rear locomotive indicated the speed at which it went off the tracks.
With about 78 passengers and five crew, the train was on a bridge over the Interstate-5 highway in Pierce County, 40m (65km) south of Seattle, when it came off the tracks at 7.40am (3.40pm GMT) on Monday.
City of DuPont fire chief Larry Creekmore said more than 100 people were taken to hospital after 13 carriages fell on to the road. The number of dead was lowered from an earlier figure of six.
A British citizen could be among the injured.
State governor Jay Inslee, visiting some of the passengers in hospital, said: “There’s a young man whose passport is on the train, going to England, and we want to make sure we can get him home and I assured him we’ll take care of that.”
An official briefed on the investigation said preliminary signs indicated the train may have struck something on the track before derailing on the Mounts Rd overpass.
Seattle Mayor Jenny Durkan tweeted: “This catastrophic derailment is horrific. My thoughts are with those injured & our first responders who rushed to the scene.
“The City of Seattle has offered our assistance & is ready to work with Amtrak, local, state, federal officials in the aftermath of this tragic incident.”
The sheriff’s office said “multiple” motorists were injured when their vehicles were hit but no fatalities have been reported.
Traffic cameras and police photos showed carriages dangling off the bridge onto the highway and others smashed onto the busy road below.
US President Donald Trump tweeted that the incident shows “more than ever” why his infrastructure plan “must be approved quickly”.
Ten minutes later he added that his “thoughts and prayers are with everyone involved” in the accident.
Passenger Chris Karnes told CBS News: “We had just passed the city of DuPont and it seemed like we were going around a curve.
“All of a sudden, we felt this rocking and creaking noise, and it felt like we were heading down a hill.
“The next thing we know, we’re being slammed into the front of our seats, windows are breaking, we stop, and there’s water gushing out of the train. People were screaming.
“The tracks for this line were supposed to be upgraded to be able to handle higher speeds,” he continued. “I’m not sure what happened at this juncture.”
The train was using the new Point Defiance Bypass route created to avoid slow curves and “single track tunnels” on the main line tracks.
The project, which began in 2010, cost $181m (£135m), a report by the Washington State Department of Transportation revealed.
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The 501 train was the first of 12 Amtrak Cascades trains launched as part of the high-speed rail service featuring a special technology which uses gravity to tilt through curves while maintaining speeds of up to 79mph.
Travel times between Seattle and Portland were promised to be cut by 25 minutes, Amtrak said.