Amtrak train 'speeding at twice the limit when it crashed'

A passenger train that crashed in Philadelphia on its way to New York may have been going up to 160km/h around a curve it was meant to take at half that speed.

At least seven people were killed when the Amtrak train derailed at Frankford Junction in Philadelphia, Pennsylvania on Tuesday night.

More than 200 people have been injured, with at least eight listed in a critical condition.

The Wall Street Journal reports investigators believe speed is the main factor in the crash, saying the train was travelling double the 50mph restriction in the curve where it detailed.

Search teams have discovered the black box, which should give more details into the speed and inputs being put into the train by the driver.

The train's conductor was injured "to some extent" in the incident but was treated and was giving a statement to police, Philadelphia mayor Michael Nutter told reporters.

The mayor said the black box was "now in the Amtrak operations centre in Delaware for analysis," adding officials did not yet have any data from the device.

Officials reiterated that not all of the 243 people on board the train, which was travelling from Washington to New York, had been accounted for, without offering a figure. They added that some people may simply have left the scene uninjured.

"It's a devastating scene," said Robert Sumwalt, a member of the US National Transportation Safety Board, which sent a team of investigators to try to determine what went wrong.

"There are many first responders out there that are working. They are carefully examining the equipment to see if there is anyone else in the rail cars."