Was the Amtrak train hit by an object?

An assistant conductor on the Amtrak train that derailed in Philadelphia on Tuesday told investigators that the train may have been hit by some type of projectile just prior to the crash.

She said she overhead a radio transmission minutes before where the engineer of a separate commuter train reported "that he had either been hit by a rock or shot at" when he passed through the same area earlier, said National Transportation Safety Board Member Robert Sumwalt.

"She also believed that she heard her engineer say something about his train being struck by something," Sumwalt said at a news briefing. "Our investigation has not independently confirmed this information, but we have seen damage to the left hand lower portion of the Amtrak windshield that we have asked the FBI to come in and look at for us."

Sumwalt's team of investigators have been gathering physical evidence and interviewing the Amtrak crew as they try to piece together why the New York-bound train accelerated 106 miles per hour before derailing on a curve of track that had a speed limit of 50 mph. Eight passengers were killed and more than 200 people were injured.

The engineer, Brandon Bostian, 32, survived, and his lawyer has said he suffered a concussion that limited his memory of the events. He was interviewed by accident investigators, who said Bostian recalls sounding the train's bell as he passed a station in north Philadelphia and nothing after that.

The train's conductor is still hospitalised and wasn't able to be interviewed, though both of the assistant conductors spoke to investigators.

A Philadelphia commuter train operated by the Southeastern Pennsylvania Transportation Authority had passed through the area about 15 minutes before the Amtrak train and reported that an "unknown projectile" came through the engineer's window, a Septa spokeswoman, Jerri Williams, said.