An Amtrak train making the first-ever run along a new route hurtled off an overpass Monday near Tacoma, Wash., and spilled some of its cars onto the highway below, killing an undisclosed number of people, authorities said.

The Pierce County Sheriff's Office said several vehicles on Interstate 5 were struck by falling train cars and multiple motorists were injured. No fatalities of motorists were reported.

At least one rail car reportedly ended up on the highway in the derailment about 64 kilometres south of Seattle just before 7:40 a.m. PT.

The National Transportation Safety Board said a team of investigators was on its way to the scene from Washington, D.C. An NTSB spokesperson said the agency was aware of reports that it was the inaugural run of the service but wasn't able to comment further on what may have caused the crash.

APTOPIX Train Derailment Washington State

Cars from an Amtrak train lay spilled onto Interstate 5 below alongside smashed vehicles as some train cars remain on the tracks above Monday in DuPont, Wash. (Elaine Thompson/Associated Press)

Washington Gov. Jay Inslee offered condolences to the victims and declared a state of emergency.

"Today's tragic incident in Pierce County is a serious and ongoing emergency," Inslee said in a statement. "Trudi and I are holding in our hearts everyone on board, and are praying for the many injured. They are our top priority, and I know first responders are doing everything to ensure everyone has the care they need."

Inslee said officials were working on opening alternate routes to help travellers.

There were about 78 passengers and five crew aboard, Amtrak said in a statement. The Amtrak schedule called for the train to leave Seattle around 6 a.m. and arrive in Portland about 3½ hours later.

'Cars everywhere'

A member of the train's crew told an emergency dispatcher that the train came around a corner before the bridge and then "we went on the ground."

After the dispatcher asked whether everyone was OK, the crew member replied, "I am still figuring that out. We got cars everywhere and down onto the highway."

The dispatcher also asked for the driver's location, and he responded, "As soon as I know exactly where all of my train is, I'll let you know."

Amtrak crew member's call to dispatch0:47

Passengers pinned under wreckage

The train was making the inaugural run on the new route as part of a $180.7-million US project designed to speed up service by removing passenger trains from a route along Puget Sound that's bogged down by curves, single-track tunnels and freight traffic.

Daniel Konzelman, 24, was driving parallel to the train on his way to work as an accountant in Olympia. He was about 30 seconds ahead of the train on the freeway when he saw it derail.

Amtrak derailment scene LIVE0:00

Konzelman, who was driving with a friend, said they pulled off the freeway and then ran down along the tracks and over the bridge to get to the scene. 

They climbed into train cars and found people hurt — some pinned underneath the train, others who appeared to be dead, he said. If they were mobile and seemed stable, he helped them climb out. If they appeared seriously hurt, he tried to comfort them by talking to them.

"I just wanted to help people because I would want people to help me," he said. "I'm an Eagle Scout. I have a lot of first-aid training and emergency response training."

derailment-map-graphic

Amtrak train 501 derailed about 64 kilometres south of Seattle before 7:40 a.m. PT, spilling at least one train car onto busy Interstate 5. (CBC News)

Greg Mukai told CBC News that first responders arrived at the scene quickly and that military personnel from the nearby Joint Base Lewis-McChord also descended on the scene to offer help.

Track testing conducted

Track testing was completed in January and February in advance of Monday's launch, according to the Washington State Department of Transportation.

Don Anderson, mayor of Lakewood, Wash., a city along the new route, predicted a deadly crash, albeit one involving a fast-moving train hitting a car or pedestrian at a grade-crossing. At a recent public meeting, he called on state planners to build overpass-like rail structures instead of having trains cross busy streets.

Transitdocs.com, a website that maps Amtrak train locations and speeds using data from the railroad's train tracker app, says Train 501 was going 130 kilometres per hour moments before the derailment.

Local law enforcement officials have not provided specific numbers on the dead and injured. But hospital officials say at least two people are in critical condition and 11 others are seriously injured.

Four hospitals say 50 people have been hospitalized but didn't immediately report all of their conditions. At least one person was in surgery Monday.

Medical tents were set up in a median area, just south of DuPont.

Amtrak's statement said individuals with questions about their friends and family on Train 501 should call 1-800-523-9101.

President Donald Trump on Twitter said his thoughts and prayers were with anyone affected by the incident, several minutes after a first tweet that said the derailment "shows more than ever why our soon to be submitted infrastructure plan must be approved quickly."

Train Derailment Washington State

This image from video provided by KOMO-TV gives an aerial view of the Amtrak train that derailed south of Seattle near Tacoma. (KOMO-TV via AP)

With files from CBC News and Reuters