Amtrak train was traveling at more than 100 miles per hour when it derailed
Emergency workers look through the remains of a derailed Amtrak train in Philadelphia, Pennsylvania on May 13, 2015. Photo by Lucas Jackson for Reuters.

An Amtrak train in Philadelphia was traveling at more than 100 miles per hour, over twice the speed limit, when it entered a curve in the tracks and derailed, killing seven people and injuring more than 200, federal investigators said on Wednesday.

While the precise cause of Tuesday night's crash 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 applied the locomotive's emergency braking system just after entering the curved stretch of track, where the maximum allowed speed is 50 miles per hour (80 km per hour), Sumwalt said.

But the brakes managed to only slightly slow the train to 102 mph from 106 mph (171 kph) before the locomotive and all seven passenger cars derailed, he said.

Authorities have offered no explanation for why the locomotive was traveling at more than double the authorized 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."

He said the data collected by black box event recorder recovered from the wreckage had yet to be fully analyzed, and investigators also planned to pore over video footage from forward-facing cameras attached to the train.

He said the NTSB team expected to remain on the crash scene for about a week.

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

The commuter rail route where the Amtrak train left the track was not governed by positive train control technology, which is designed to prevent high-speed derailments, officials said.

Passenger rail service along Amtrak's Northeast Corridor, the country's busiest with 12 million passengers a year, was shut down immediately after the accident at about 9:30 p.m. EDT Tuesday (0130 GMT Wednesday), leaving travelers scrambling for alternatives. The derailment also snarled commuter rail services that share Amtrak tracks in the Philadelphia area and beyond.

Nearly a day after Amtrak No. 188 jumped the track, rescue workers were still pulling apart the twisted metal and sifting through other debris left by the crash. One of the seven cars landed upside down and three were tossed on their sides, while passengers and luggage were sent flying, survivors said, inflicting severe injuries on some of them.

Philadelphia Mayor Michael Nutter said at a news conference that seven people were confirmed dead, but authorities had not yet accounted for everyone believed to have been on board. He said rescue teams expanded the search area out of fear that some victims may have been thrown from the train when it derailed.

Positive train control (PTC) automatically slows or even halts trains that are moving too fast or heading into a danger zone. Under current law, the rail industry must adopt the technology by year-end.

But the crash came a day before the House Appropriations Committee approved a transportation budget for the next fiscal year that included a funding cut for Amtrak.

One amendment proposed by Democrats called for $825 million in capital investments in PTC technologies for passenger rail, but it was blocked by the Republican majority.

(Reporting by Jarrett Renshaw; Additional reporting by Ellen Wulfhorst, Laila Kearney and Ryan McNeill in New York, and Susan Heavey in Washington; Writing by Frank McGurty; Editing by Jeffrey Benkoe and Lisa Shumaker)