However, investigators say they are not yet sure what caused the damage.
That area came under scrutiny after reports that two other trains, a SEPTA train and an Acela train, were hit by unknown objects in the minutes before the Amtrak crash on Tuesday night.
It is still not yet known if the incidents are connected.
The FBI found no evidence of damage to #Amtrak 188 windshield that could have been caused by a firearm.— NTSB (@NTSB) May 18, 2015
NTSB has not ruled out possibility that another object may have struck the #Amtrak locomotive windshield.— NTSB (@NTSB) May 18, 2015
Meanwhile, two Center City law firms joined forces to announce the filing of the first lawsuits against Amtrak on behalf of four seriously injured passengers aboard the train.
Among the plaintiffs are a Spanish retiree who almost had her arm severed and a New York advertising executive who was in the first car that overturned.
"Somehow he ended up below the train, he's not exactly sure why. He fractured his ribs, bruised his lung and knocked out his teeth," said attorney Robert Mongeluzzi.
The lawyers chastised Amtrak for not installing an Automatic Train Control, a speed control device, on the northbound curve until after the fatal crash.
They also had harsh words for the 32-year-old engineer.
"The conduct of the engineer is both unfathomable and unconscionable," said attorney Thomas Kline.
This comes the same day that Amtrak trains began rolling again between Philadelphia and New York early Monday, the first time since the derailment.
Amtrak resumed service with a 5:30 a.m. southbound train leaving New York City. The first northbound train, scheduled to leave Philadelphia at 5:53 a.m., was delayed and pulled out of 30th Street Station at 6:07 a.m.
"I'm confident we're going to be safe," said Mary Schaheen.
Action News was overhead and on the train, broadcasting live video as the train departed Philadelphia.
Both trains arrived at their destinations about 30 minutes behind schedule.
About 60 people boarded the New York-bound train in Philadelphia.
"It's somewhat harrowing to be on first train back up there, but everyone has absolutely been doing their job," said Tom Carberry.
"I'm a little nervous, my family's a little nervous for me, but I have to get back to New York," said Chris Britton.
Mayor Michael Nutter was on hand to see the train off.
"I'm in Philadelphia today, but if I needed to go to New York today, I'd be on this train," Nutter said.
President Barack Obama, landing in Philadelphia before a visit to nearby Camden, New Jersey on Monday, thanked the city and its rescue workers for their response to the derailment.
He shook hands with Nutter, police Commissioner Charles Ramsey and other city officials after arriving at Philadelphia International Airport on Air Force One.
On Saturday, federal regulators ordered Amtrak to expand the use of their automatic train control system to northbound trains near the crash site. That's now fully operational on the northbound tracks. It has long been in effect along that southbound stretch.
"What that will do is basically provide an assurance that the train will come to stop if there's a violation of speed in that area," said Amtrak spokesperson Craig Schulz.
All Acela Express, Northeast Regional and other services also resumed.
Amtrak said there are some minor repairs that need to be completed in the area of the crash, but those repairs wont' stop the trains from running.
(NOTE: An earlier version of this story said sources told us 'human error' was to blame. However, questions about that information have since arisen and the story has been changed.)