A train making the first-ever run along a faster new route has hurtled off an overpass south of Seattle and spilled some of its cars onto the highway below, killing at least three people, injuring dozens.

Attention quickly turned to the Amtrak train’s speed. Federal investigators say the train was travelling at 80m/h (129km/h) in a 30m/h (48km/h) zone.

Bella Dinh-Zarr, an NTSB board member, said at a Monday night news conference that information from the event data recorder in the rear locomotive provided information about the train’s speed.

Dinh-Zarr said 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.

There were 80 passengers and five on-duty crew on board when the train derailed and pulled 13 cars off the tracks. Authorities said there were three confirmed deaths. More than 70 people were taken for medical care – including 10 with serious injuries.

About two hours after the accident, a US official who was briefed on the investigation said he was told at least six people were killed. The official said he had no new information to explain the discrepancy in the numbers.

The official was not authorised to discuss the investigation publicly and spoke to The Associated Press on condition of anonymity.

A track chart prepared by the Washington State Department of Transportation shows the maximum speed drops from 79m/h (127km/h) to 30m/h for passenger trains just before the tracks curve to cross Interstate 5, which is where the train went off the tracks.

The chart, dated February 7, 2017, was submitted to the Federal Railroad Administration in anticipation of the start of passenger service along a new bypass route that shaves 10 minutes off the trip between Seattle and Portland.

In a radio transmission immediately after the accident, the conductor can be heard saying the train was coming around a corner and was crossing a bridge that passed over Interstate 5 when it derailed. Dispatch audio also indicated that the engineer survived with bleeding from the head and both eyes swollen shut.

“I’m still figuring that out. We’ve got cars everywhere and down onto the highway,” he tells the dispatcher, who asks if everyone is OK.

Aleksander Kristiansen, a 24-year-old exchange student at the University of Washington from Copenhagen, was going to Portland to visit the city for the day.

“I was just coming out of the bathroom when the accident happened. My car just started shaking really, really badly,” he said.

The back of his train car was wide open because it had separated from the rest of the train, so he and others were able to jump out to safety. He was at about the middle of the train, either the sixth or seventh car, he said, and was “one of the lucky ones”.

By Rachel La Corte, Gillian Flaccus and Michael Sisak