By Katherine Skiba, Tribune Washington Bureau (TNS)

WASHINGTON — Full Amtrak service has resumed in the nation’s busy Northeast rail corridor, almost a week after a derailment in Philadelphia killed eight passengers and injured more than 200.

Since the May 12 crash of an Amtrak train headed from Washington to New York City, Amtrak officials “have been working around the clock” to make repairs that would allow full service to resume through Philadelphia, Amtrak President and Chief Executive Joe Boardman said Sunday in a statement.

The possibility that a rock or other projectile hit Amtrak Northeast Regional Train 188 moments before it derailed is one of many that safety officials are exploring as they seek a cause for the deadly accident, a top federal official said Sunday.

The FBI will be at the accident scene Monday to examine the wreckage and try to determine what may have hit the train, said Robert Sumwalt, a member of the National Transportation Safety Board.

So far, officials have interviewed dispatchers and listened to dispatch tapes and have “heard no communication at all from the Amtrak engineer to the dispatch center to say that something had struck his train,” Sumwalt said as he made the rounds of Sunday morning news programs. Nor did the engineer of a nearby commuter train that was struck recall any conversation between the crew of his train and Amtrak 188, he added.

“We’re just in the fact-finding stage of the investigation,” he said. “We’re just slowly starting to gather the information and then slowly start ruling things out.”

Sumwalt said crew members on the commuter train operated by the Southeastern Pennsylvania Transportation Authority reported being hit by a projectile. The NTSB does not know how many other trains had been struck, he said.

Sumwalt, a former airline pilot, said the investigation would have been “significantly” helped if the Amtrak train had been equipped with inward-facing video cameras.

Sumwalt urged that advanced technology, known as positive train control, be implemented soon to avoid future derailments. He called it “very troubling” that positive train control might not be installed on passenger railways until year’s end.

Positive train control utilizes GPS technology to monitor a train’s location and can enforce speed limits.

“We have seen countless accidents over the years that could have been prevented had positive train control been implemented,” Sumwalt said.

After the crash of a Metrolink commuter train in Southern California seven years ago, Congress ordered the nation’s rail operators to install positive train control by the end of this year. Progress toward that goal has been slow.

Amtrak is closer to achieving the goal than many other railroads, with the train controls installed on parts of the Northeast Corridor between Boston and Washington, but not in the section where the derailment happened. Most of the nation’s freight lines have lagged behind, and several have asked Congress for an extension of the deadline.

Federal regulators have ordered Amtrak to install a less advanced system, known as automatic train control, before reopening the line through the area where the derailment took place. Amtrak officials have said they will do so.

(Staff writer Matt Pearce in Los Angeles contributed to this report.)

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Photo: John H Gray via Flickr