An Amtrak train that derailed in Philadelphia, killing seven people, was travelling into a curved stretch of track at more than 160kph, over twice the speed limit, when the engineer slammed on the brakes, investigators have said.

While the cause of Tuesday night's crash that injured more than 200 people remains to be determined, experts from the National Transportation Safety Board (NTSB) believe the derailment would have been prevented by installation of an advanced safety system called "positive train control", NTSB board member Robert Sumwalt said.

The engineer of the New York City-bound passenger train fully applied the locomotive's emergency braking system as it entered the left-hand curve in the rail line, where the maximum allowed speed is 80kph, Sumwalt told a news conference hours after the NTSB began its probe.

But the brakes managed to only slightly slow the train from 171kph to 164kph in the few seconds before the locomotive and all seven passenger cars derailed, he said.

Asked how long it should take to slow a train moving that fast to the posted speed, Sumwalt said: "He was already in the curve at that point. You're supposed to enter the curve at 50 miles per hour [80.5kmp]."

Philadelphia Mayor Michael Nutter reacted with dismay to the disclosure, calling it "astounding, devastating".

Authorities have offered no explanation for why the No 188 train, which originated in Washington DC, and had stopped in Philadelphia's 30th Street station about 20 minutes earlier, was travelling at more than double the authorised speed.

Sumwalt said NTSB investigators had not yet interviewed the engineer, who was injured in the wreck, but planned to do so in the next couple of days.

"This person has gone through a very traumatic event and we want to give him an opportunity to convalesce for a day or two," Sumwalt said. "But that is certainly a very high priority for us."

Nutter has said the engineer gave a statement to police, though officers have declined to comment.

Sumwalt said data collected by the "black box" recorder recovered from the wreckage had yet to be fully analysed, and investigators also planned to examine video footage from forward-facing cameras attached to the train.

He said the NTSB team expected to remain on the crash scene, in the Port Richmond section of north Philadelphia along the Delaware River, for about a week.

In addition to speed, the NTSB has said it was focusing on the condition of the tracks and signalling equipment, crew training and the performance of the five-person crew.

Source: Agencies