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An Amtrak train that derailed in Philadelphia, killing at least seven people, was travelling at twice the speed limit, say safety experts, the BBC reports.

The driver applied the emergency brakes when the train hit 106mph (170km/h) on a 50mph track, said the National Transportation Safety Board (NTSB).

But his efforts had only brought the speed down to 102mph when the deadly crash happened.

The speed was recorded in the so-called black box recovered from the wreckage.

Robert Sumwalt of the NTSB told reporters a speed control system in place along parts of that route along Amtrak's Northeast Corridor was not yet in place on that section.

"We feel that had such a system been installed in this section of track, this accident would not have occurred," he said.

Amtrak Train 188 was going from Washington to New York when it derailed on Tuesday night, leaving more 200 people injured.

The death toll rose from six to seven on Wednesday, as another body was found by the search and rescue team.