DUPONT, Wash. — An Amtrak train has derailed on an overpass in Washington state, with at least two train cars falling onto the highway below. Authorities say multiple fatalities and injuries have been reported. At this time, all southbound lanes of traffic remain blocked on Interstate 5. 

The Washington State Department of Transportation tweeted a photo of the derailment, urging motorists to avoid the area.

CBS News affiliate KIRO-TV reports first responders are treating the derailment as a mass casualty incident. Authorities have not yet provided details on injuries.

“Emergency crews are on the scene and the Amtrak Management is responding,” an Amtrak spokesperson said in a statement. “Some injuries are reported. Service between Seattle and Portland is suspended for the time being.”

Amtrak train 501, which departed shortly before 8 a.m. local time from Tacoma, was part of a new service that launched Monday morning. The train is able to carry up to 250 people, but officials said there were about 75 aboard at the time of the crash.

The Cascades service from Seattle to Portland was supposed to be rated for a maximum speed of 79 mph for the section where it derailed in DuPont, according to passenger Chris Karnes.

Karnes, who was on board the train when it derailed, said at least seven cars derailed from the tracks. The emergency doors were not functioning correctly, which forced passengers to kick out train windows to exit.

“We had just passed the city of DuPont and it seemed like we were going around a curve,” Karnes said. “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 added. “I’m not sure what happened at this juncture.” CBS News correspondent Kris Van Cleave reports the train tracks themselves are not owned by Amtrak; they’re maintained by a freight railroad.

A photo taken by a Washington state trooper showed one car of the train smashed upside down on the highway and another hanging off the bridge above it.

The cause of the derailment remains under investigation.