FBI experts to examine Amtrak train's windshield

Thousands of commuters who depend on Amtrak's Northeast Corridor were once again boarding trains Monday. Amtrak resumed service between Philadelphia and New York in the morning -- not quite a week after the deadly derailment that killed eight and injured more than 200.

\"The safety of our passengers and crew remains our number one priority,\" said Amtrak President and CEO Joe Boardman in a written statement Sunday. \"Our infrastructure repairs have been made with the utmost care and emphasis on infrastructure integrity, including complete compliance with Federal Railroad Administration directives.\"

The first trains rolled out of Philadelphia and New York City before 6:00 a.m. Amtrak says all Acela Express, Northeast Regional and other services will also resume.

Did something hit the windshield?

The National Transportation Safety Board's investigation into last Tuesday's accident continues. The NTSB is tapping FBI experts to investigate whether a mark on the windshield of Amtrak Northeast Regional Train 188 was made by a hurled projectile -- or even a bullet -- before it derailed in Philadelphia last week.

\"The FBI will be on the scene (Monday) to assist us to identify what that may have been,\" NTSB board member Robert Sumwalt told CNN's \"State of the Union\" on Sunday.

At least two other trains -- a regional Southeastern Pennsylvania Transportation Authority train and an Amtrak Acela -- reported being struck with projectiles in the area near the crash site.

Sumwalt expressed some reservations about reports something struck Train 188 before it derailed.

\"We did listen to the dispatch tapes between dispatch and the trains, and indeed the SEPTA engineer did report to dispatch that he had been struck by something. But there was nothing, nothing at all from the Amtrak engineer to dispatch to say that his train had been struck,\" he told CNN's Brianna Keilar.

\"Furthermore, we have interviewed the SEPTA engineer. And he did not recall having any conversation between him and the Amtrak engineer. But, nevertheless, we do have this mark on the windshield of the Amtrak train, so we certainly want to trace that lead down.\"

SEPTA passenger Alfred Price told CNN he heard a loud boom before the train he was riding on came to a stop, and the engineer, who appeared shaken, told passengers something had hit the train. A photo of the front of the SEPTA train shows a circular crack on the windshield.

Kam Desai was a passenger on the Acela that was about 20 minutes ahead of Train 188 when something struck and cracked the side window on the row behind her. \"We heard a very large, really loud slamming or banging sound,\" Desai said. \"It was very alarming to all the passengers, myself included, and my co-worker that was with me.\"

New safety measures ordered

Meanwhile, Amtrak spent the weekend installing new speed controls on the curved section of track at Frankford Junction where the fatal derailment occurred, the result of an order by the Federal Railroad Administration to install the Automatic Train Control system as an immediate step to improve safety.

ATC has been in place throughout the Northeast Corridor, the most heavily traveled rail network in the country, for nearly 40 years. The system notifies an engineer if a train is speeding and applies the brakes automatically if the engineer does not respond.

The system is in place at Frankford Junction for southbound trains, which enter the 50-mph curve from a maximum speed of 110 mph, Amtrak says. But it's not in place for northbound trains, which enter from a maximum speed of 80 mph.

Amtrak 188 was traveling northbound at 106 mph when it entered the curve, causing it to careen off the tracks so violently that three of the seven cars that derailed were left standing upright.

\"Had the train been operating at max authorized speed heading into the curve, it would not have come off the tracks,\" Amtrak wrote.

The FRA also instructed Amtrak to assess the risk of all curves on the corridor where the approach speed is significantly higher than curve speed, and to increase speed limit signage throughout the corridor.

Amtrak said it would immediately implement the measures.

Better speed controls by year's end

Amtrak is in the process of installing a sharper technology known as Positive Train Control on all of its tracks. The PTC system is already in service between Boston and New Haven, Connecticut, but is in service on only 50 of the 226 miles of track between Washington and New York.

It is not installed at Frankford Junction.

PTC is a programmable system that uses transponders in the tracks to communicate with computers on locomotives. As a train passes over a transponder, it switches the train's onboard radio to the proper channel and helps the train receive the appropriate information about speed restrictions and routes, according to Amtrak.

As with ATC, the system sends a warning to the engineer if the train is speeding and applies the brakes if the engineer doesn't respond.

Congress ordered the nation's railroads to adopt PTC by December 2015 in response to a head-on collision that killed 25 people in 2008 near Los Angeles. The technology is complicated and expensive, but Amtrak says it is on schedule to meet the end-of-year deadline.

Sumwalt and Boardman said the derailment would not have happened if PTC had been in place.

TM & © 2015 Cable News Network, Inc., a Time Warner Company. All rights reserved.