Amtrak will resume service Monday between New York City and Philadelphia, the site of a deadly train derailment nearly a week ago that killed eight passengers, the company said Sunday.

The company suspended service between the cities after the May 12 derailment in Philadelphia's Port Richmond neighborhood. More than 200 people were injured.

Amtrak had said it expected to restore limited service Monday, with full service resuming Tuesday.

"Amtrak staff and crew have been working around the clock to repair the infrastructure necessary to restore service for all the passengers who travel along the Northeast Corridor," Amtrak president and CEO Joe Boardman said in a news release.

Meanwhile, investigators were trying to determine the reason for the train's acceleration and were sorting through conflicting reports about an object striking its windshield, The Associated Press reported.

National Transportation Safety Board member Robert Sumwalt said on CBS' "Face the Nation" program Sunday that he wanted to "downplay" the idea that damage to the windshield might have come from someone firing a shot at the train shortly before it flew off the tracks, the AP said.

Officials said an assistant conductor on the derailed train said she heard the Amtrak engineer talking with a regional train engineer and both said their trains had been hit by objects.

But Sumwalt said the regional train engineer recalls no such conversation, and investigators had listened to the dispatch tape and heard no communications from the Amtrak engineer to the dispatch center to say that something had struck the train, the AP reported.

Sumwalt acknowledged in an interview on "Fox News Sunday" that train engines are routinely struck by projectiles without catastrophic consequences.

According to the AP, investigators remain focused on the acceleration of the train as it approached the curve, finally reaching 106 mph as it entered the 50-mph limit stretch, and only managing to slow slightly before the crash.

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The Federal Railroad Administration ordered Amtrak to expand use of a speed-control system long in effect for southbound trains near the crash site to northbound trains in the same area.

The agency also ordered the company to examine all curves along the busy stretch of tracks between Washington and Boston, and determine if more can be done to improve safety, and to increase speed limit signs along the route.

Amtrak said it would immediately comply.

Meanwhile, almost 20 people injured in the train crash remained in Philadelphia hospitals, with five of them in critical condition, but all were expected to survive.With AP