Amtrak on Sunday expressed condolences to the families of those killed when a train derailed the day before in northern Montana, saying it was "fully cooperating" with an investigation into the deadly incident.

"We have no words that can adequately express our sorrow for those who lost a loved one or who were hurt in this horrible event," the US rail operator's CEO, Bill Flynn, said in a statement.

Around 141 passengers and 16 crew members were traveling from Chicago towards the Pacific coast on Saturday afternoon when eight of the 10 cars came off the tracks at around 4:00 pm (2200 GMT), killing three people and injuring dozens more.

"We are fully cooperating with the investigation, working closely with National Transportation Safety Board, the Federal Railroad Administration, local law enforcement and response agencies," Flynn said.

"We share the sense of urgency to understand why the accident happened; however, until the investigation is complete, we will not comment further on the accident itself."

Rescuers rushed to evacuate everyone on board after the derailment, with Amtrak later saying local authorities had confirmed "that three people have lost their lives as a result of this accident," adding there were also "reported injuries."

Footage posted on social media showed people waiting by the tracks, luggage strewn next to them, with train carriages seen listing off the rails and at least one toppled onto its side.

Flynn said the company was working to support passengers and crew and had set up an incident response team to help those who had been on board the train.

It was not immediately clear what caused the derailment.

The National Transportation and Safety Board said it was launching a "go-team" to investigate the derailment.

Around 141 passengers and 16 crew members were traveling from Chicago towards the Pacific coast on an Amtrak train (similar to the one pictured in July 2021) when eight of the 10 cars came off the tracks
Around 141 passengers and 16 crew members were traveling from Chicago towards the Pacific coast on an Amtrak train (similar to the one pictured in July 2021) when eight of the 10 cars came off the tracks AFP / Daniel SLIM

Montana's Disaster and Emergency Services coordinator Amanda Frickel on Saturday told The New York Times that "well over 50 people had been injured."

She said teams of rescuers were at the scene and multiple hospitals -- as well as medical helicopters -- were on standby.

Authorities were not releasing further details for the time being, she said, adding: "Everybody who is alive has been extricated from the wreck."

The Empire Builder train derailed in a remote area near Joplin, Montana, a town of around 200 people near the border with Canada.

Montana Senator Jon Tester said he was monitoring the derailment.

"My thoughts are with all the passengers, crew, and first responders on the scene," he added.

Amtrak said it was canceling some trains on Saturday between the states of North Dakota and Montana, and abridging two Empire Builder train services on Sunday, with no substitute transit available.

The US rail network suffers from chronic underfunding, and fatal accidents sometimes occur.

In 2018, two people died in South Carolina when an Amtrak train traveling on the wrong tracks collided with a stationary freight train, in a crash later blamed on safety oversights.

A year earlier, an Amtrak passenger train traveling on a new route for the first time derailed in Washington state, killing at least three people as cars plunged off a bridge onto a busy highway at the height of morning rush hour.