Investigators say a US Amtrak train that derailed south of Seattle, killing three people and injuring dozens more, was travelling at 128km/h (80m/h) in a 48km/h (30m/h) zone.

Bella Dinh-Zarr, an National Transportation Safety Board board member, said at a news conference that information from the event data recorder in the rear locomotive provided information about the train's speed.

Authorities say at least three people were killed and dozens injured when the train derailed early this morning, spilling train cars onto a busy interstate highway.

Ms Dinh-Zarr says it's not yet known what caused the train to derail and that "it's too early to tell" why it was going so fast.

Cars from an Amtrak train that derailed above lay spilled onto Interstate 5 alongside smashed vehicles. (AAP)

Seventy-seven passengers and seven crew members were on board the new, high-speed commuter train when it hurtled off an overpass near DuPont before 8am local time (3am Tuesday AEDT).

The train had left Seattle at 6am local time and was en-route to Portland.

Authorities said a total of 13 cars jumped the tracks, with a number of carriages falling on to the Interstate-5 highway below.

According to Washington State Patrol spokeswoman Brook Bova, five vehicles and two semi-trailers were involved in the accident, but none of the fatalities were motorists.

More than 100 people have been taken to area hospitals, with two of those injured critical and 11 seriously injured.

Trucks and cars were tangled in the devastation from the truck crash, (AAP)

Several vehicles were destroyed when the train derailed. (AAP)

One survivor described the moment he realised the train was going to derail.

“Glass shattered and people were flying,” he said.

“For some reasons, someone just told me to buckle up, so I did and I got underneath my table.

“At that time, we were still moving and I was thinking, anything could happen.

“You think, ‘it is ever going to stop?’ Then it did. And when it stopped, it was completely black.”

The train can be see danging off a bridge. (AAP)

A passenger train is dangling over a motorway in the US, with fears it may be a "mass casualty incident". (AAP)

The train was making the first journey on a section of track that was recently upgraded for $181 million.

Chris Karnes, a member of the Community Transportation Advisory Group, had taken the train to experience its inaugural run.

He described hearing “creaking” in the moments before the crash before the train “started to wobble for a moment”.

“Then we were catapulted at the seats in front of us and the next thing we know, our car had crumpled at a portion at the top and we were down an embankment.”

It is not known how many people are injured. (AAP)

At least 77 people have been injured. (Pierce County Sheriff's Department)

Mr Karnes’ car, about three or four from the back and carrying about ten passengers, then “came to a rest on a hill”.

"We could hear and feel the cars crumpling. Water was coming from the ceiling and there was a lot of dust.

"We had to kick out the emergency windows because the emergency doors weren't functioning.

"The only car that is on the tracks is the rear locomotive. There are several cars that are hanging over the overpass."

US President Donald Trump has offered his "thoughts and prayers" for those involved in the train derailment in Washington state.

But he wasted no time scoring political points.

Ed Troyer, a spokesman with the Pierce County Sheriff's office, earlier said: "At this point we’re just told there’s multiple fatalities."

"The people that were in all the vehicles, even though when you see the pictures it’s pretty horrific, at this point nobody in any of the vehicles is a fatal."

Mr Troyer said the priority at the scene for emergency services was to "get the survivors out, get the injured to the hospitals".

The National Transportation Safety Board is investigating the incident, with a team of 20 investigators making their way to the scene to retrieve the train’s black box.

Wreckage on the road at the scene of the derailment. (Pierce County Sheriff's Department)

The train derailed just after 8am local time. (Pierce County Sheriff's Department)

The train left Seattle around 6am local time and was heading to Portland. (9NEWS)

Investigations will consider the speed of the train and possible obstructions on the track, but “anything beyond that is pure speculation”, according to an Amtrak spokeswoman.

Amtrak earlier confirmed the train that derailed was Train 501 from Seattle to Portland.

It left Seattle around 6am Monday local time (1am Tuesday AEDT), according to an Amtrak schedule, and was due in Portland about three-and-a-half hours later.