The NTSB has been calling for "positive train control" for years, and federal law requires that it be installed by the end of this year on all passenger railroads and major freight carriers.
But many railroads have asked for more time to install the expensive systems, and Congress is considering extending the deadline to 2020.
"If the NTSB has indicated that this was preventable from a variety of standpoints, then it's even more egregious," Mayor Nutter said Wednesday night.
In a statement, President Obama commended first responders and medical personnel, as well as uninjured passengers, who assisted the injured.
"Along the Northeast Corridor, Amtrak is a way of life for many," he said. "From Washington, D.C., and Philadelphia to New York City and Boston, this is a tragedy that touches us all."
The incident marked the deadliest accident on the Northeast Corridor since 16 people were killed when an Amtrak train collided with two Conrail locomotives near Baltimore in 1987.
In Tuesday's crash, the engineer apparently applied full emergency brakes just seconds before the train derailed, slowing the train from 106 to 102 m.p.h., Sumwalt said.
The speed limit for the sharp curve where the accident occurred is 50 m.p.h. The speed limit before the bend is 80 m.p.h.
The train's conductor, who was not identified, was at Einstein Medical Center with a skull fracture, Police Commissioner Charles H. Ramsey said Wednesday.
The train's engineer, identified by law enforcement as Brandon Bostian, 32, of New York City, was interviewed by police, Nutter said at an evening news conference. He said he did not know the details of that conversation.
"It is clearly, at a minimum, irresponsible to have a train going so fast," Nutter said.
Bostian, however, declined to give a statement to police investigators and left the East Detectives Division with an attorney, police commissioner Charles H. Ramsey said Wednesday.
According to his LinkedIn page, Bostian has worked as an Amtrak engineer since December 2010. He moved up to that job after working as an Amtrak conductor for the previous 41/2 years. He attended business school at the University of Missouri in Columbia.
After the crash, Amtrak employees, railroad buffs, and others left about three dozen messages of concern on Bostian's Facebook page. A man who identified himself as an Amtrak engineer urged Bostian to ignore negative news stories.
"Every day we hold lives in our hands - 99.9% of the time it goes unappreciated and taken for granted," he wrote. "Yes, it happened to you but it could have been any one of us and you are not alone."
While messages continued to surface on the page, there were no replies posted by its owner. The main photo on the page was taken down - replaced with a blank box - early Wednesday, about five hours after the crash.
Nutter said the train's event recorder was recovered and was being examined at an Amtrak facility in Delaware. The train also was equipped with a forward-looking video camera that will be part of the investigation.
"We are heartbroken by what has happened here," Nutter said. "We have not experienced anything like this in modern times."
Gov. Wolf said of the destruction: "It was awful. I've never seen anything like that in my life." Asked what he thought about the train's speed, he said, "It's too fast. Strikes me as being too fast."
To prevent such accidents, Amtrak crews have been installing "positive train control" systems on the Northeast Corridor and other Amtrak routes, and they were to install the system this year in the Philadelphia area.
The system is designed to automatically slow speeding trains, prevent collisions between trains, and enforce speed restrictions. With the system, an onboard computer activates a train's brakes if the engineer doesn't.
"Based on what we know right now, we feel that had such a system been installed in this section of track, this accident would not have occurred," Sumwalt said.
Without positive train control, "everybody on a train is one human error away from an accident," the safety board said last year in renewing its call for prompt installation of PTC.
"Each death, each injury, and each accident that PTC could have prevented, testifies to the vital importance of implementing PTC now," the board said.
On much of the Amtrak network, including Tuesday's crash area, Amtrak relies on "automatic train control," an older system that establishes safe braking distances but does not control speeds as precisely as PTC.
Wolf ordered all Pennsylvania flags in the Capitol Complex in Harrisburg and at state offices throughout Pennsylvania facilities statewide to fly at half-staff until sundown Sunday to honor the victims of the derailment.
One of the dead was identified as Associated Press employee James Gaines, 49, a father of two, who died at Temple University Hospital. He had been returning home to Plainsboro, N.J., after being in Washington for meetings.
The U.S. Naval Academy said a midshipman on leave was among the dead. His family identified him as Justin Zemser, a 20-year-old from New York City, who was in his second year at the academy. In a statement, his parents said: "This tragedy has shocked us in the worst way and we wish to spend this time grieving with our close family and friends."
A third victim was identified as Abid Gilani, 55, a Wells Fargo senior vice president who worked in the bank's commercial real estate division.
Wednesday night, the family of Rachel Jacobs, chief executive of an online-learning start-up in Philadelphia, released a statement saying she had died in the accident.
Late Wednesday, a fifth victim was identified as Derrick E. Griffith, acting dean of student affairs at Medgar Evers College, part of the City University of New York. A posting on the Facebook page of CUNY Prep, a high school of which Griffith was a founding director and principal, said the school community was "struggling with this terrible news and trying to find our way during this sad time."
At least eight passengers remained in critical condition Wednesday night. Among the seriously injured was Eli Kulp, a nationally prominent, award-winning chef of the Old City restaurants Fork and High Street on Market, and a.kitchen on Rittenhouse Square.
Maryland business executive Robert Gildersleeve, 45, was among the unaccounted for, and members of his family traveled to Philadelphia to try to find him.
"We went to all the hospitals several times, and Bobby's brother has been on the scene for the last 15 hours," said his sister, Doreen DeMarco, 47, of Holmdel, N.J.
Gildersleeve's daughter, Ryan, 15, and son, Marc, 13, distributed fliers bearing their father's picture outside the Marriott Downtown Hotel on Market Street, where the Red Cross had set up a center for families.
Herbert Cushing, Temple's chief medical officer, said the hospital had received 54 patients, 22 of whom remained hospitalized Wednesday. He said most of the injured had suffered broken ribs, which indicated "that they rattled around in the train car a lot."
Among the injured was Caleb Bonham, 28. He was in the last car, listening to music on his iPod, when he noticed his computer start shaking. Suddenly "everything went black," he said.
"Next thing you know, you wake up and you're on the other side," he said. He was thrown to the opposite side of the train car.
"My recollection of what happened . . . it's really hazy," he said.
Passengers' belongings were scattered all over, he said. Even the train seats had tumbled around the car. "There was stuff flying all over the place."
Bonham said he hit his head but felt OK. After the accident, he and other passengers began to help others off the train.
Many people in his car could stand up on their own. One woman had a broken leg. He saw one woman who had lost her front teeth and was bleeding from her mouth.
"Blood was all over the place," he said. He exited through the back door of the train car.
Police directed Bonham and others who were able to walk but needed medical attention to a SEPTA bus. He said he was transported to Aria Health-Torresdale Campus, where he waited to have his minor head injury checked, and was released about 2 or 3 a.m. He left and got a hotel room for the rest of the night.
At 30th Street Station, two dozen American Red Cross volunteers wearing red vests were on hand Wednesday in response to the derailment.
Congress acted in 2008 to require positive train control after a deadly collision in Chatsworth, Calif., in which a Metrolink commuter train collided head-on with a Union Pacific freight train, killing 25 and injuring more than 100.
Investigators found the engineer of the Metrolink train was texting and ran past a red stop signal before crashing into the oncoming freight train.
Amtrak said it hoped to have PTC in place on the corridor by the end of this year. Currently, the system is operational on the corridor between Boston and New Haven, Conn.; New Brunswick, N.J., and Trenton; and Perryville, Md., and Wilmington.
In the Philadelphia area, SEPTA is spending more than $300 million to install PTC on its commuter rail network, and the transit agency expects to meet the federal year-end deadline, deputy general manager Jeffrey Knueppel said Wednesday.
"We put this as our highest priority," Knueppel said. "We've moved ahead as fast as we could do this."
U.S. Sen. Pat Toomey (R., Pa.), who went to the accident scene Wednesday, declined to say whether he would support a delay in requiring PTC. "I want to understand what we have now, what it's capable of, and what the alternative would accomplish for us," Toomey said.
In recent years, Toomey has supported a delay. In November 2013 he signed on as one of 15 cosponsors to a bill to push back a deadline for implementing PTC to 2020.
In 2012, Toomey wrote then-Transportation Secretary Ray LaHood arguing for a delay.
"There is simply no money available to pay for it without unnecessary budgetary trade-offs," Toomey wrote, "especially when considering that no extraordinary safety need will be met by rushing toward this implementation date."
In an interview Wednesday night, Toomey said that he supported implementing PTC on Amtrak by the end of this year, the current deadline, and that his previous concerns were about forcing SEPTA to quickly spend money on a new system - possibly stalling other safety investments - when it already had its own safeguards.
U.S. Rep. Robert Brady (D., Pa.) said after visiting the site that he would call for a federal audit of all rail systems to see which are compliant with implementing PTC and which are not, noting that the requirement was passed in 2008. "We gave them seven years," he said. "If that was done, this train accident would not have happened."
Tuesday's crash accident happened at a sharp curve in Frankford, near Port Richmond, where several rail lines merge. Trains are restricted to 50 m.p.h. there.
The speeding Train 188 failed to make the turn and derailed, scattering its locomotive and cars across the tracks of the Northeast Corridor and a freight line to the Delair Bridge over the Delaware River.
Tuesday's accident halted Amtrak service between Philadelphia and New York City, as well as SEPTA commuter rail service on its Trenton line. It could be a week before the SEPTA service resumes, officials said Wednesday, while Amtrak service might be restored more quickly.
Contributing to this article were Inquirer staff writers Michael Boren, Ben Finley, Joseph A. Gambardello, Matt Gelb, Caitlin McCabe, Craig R. McCoy, Laura McCrystal, Tricia Nadolny, Mike Newall, Chris Palmer, Jessica Parks, Dylan Purcell, Jonathan Tamari, Aubrey Whelan, and Martha Woodall. Philly.com staff writer Mike Klein also contributed to this article.