Why did the train accelerate as it approached a curve? Did the engineer cause the train to speed up, or was there a mechanical failure? Was there something about the track that caused the crash?
"What I believe is a very good way to interview people is, honestly, to not ask them questions, to basically give them a figurative blank sheet of paper and ask them what they recall," Sumwalt said Thursday. "Really, we want to know his account of what he recalls leading into this tragic accident."
Investigators are looking at a "good quality video" that shows the train speeding up in the moments leading up to its derailment. They don't know yet what caused the train to accelerate to more than 100 mph. Sumwalt said 65 seconds before the end of the recording, the train speed went above 70 mph, and then steadily increased.
"It just shows the speed alone," Sumwalt said. "It doesn't tell how the speed got there."
'No recollection' of crash
Bostian's lawyer told ABC's "Good Morning America" his client "has absolutely no recollection whatsoever" after losing consciousness in Tuesday night's crash.
"He remembers coming into the curve (and) attempting to reduce speed," attorney Robert Goggin said. "... The last thing he recalls is coming to, looking for his bag, getting his cell phone, turning it on and calling 911."
Initial data show the train barreled into a curve at about 106 mph, Sumwalt said. That's more than twice the 50-mph speed limit for the curve, and above the 80-mph limit immediately before it.
The engineer can't recall engaging the emergency brake, even though Sumwalt has said he did so "just moments" before the train derailed. Goggin thinks his client's memories may return as he recovers from a concussion. Bostian has 15 staples in his head, stitches in one leg and his other leg immobilized, according to his lawyer.
Goggin insisted his client hadn't been talking or texting on his phone before he made the 911 call. Nor did he have other notable accidents or mishaps. And his lawyer said Bostian voluntarily took a blood test and there was "no drinking, no drugs, no medical conditions. Nothing."
Mayor Michael Nutter said the engineer did "a pretty short interview (then) indicated that he didn't want" to talk more. The mayor noted Bostian survived after his engine car "tumbled over and over" and that he doesn't have to offer more information right away.
"He doesn't have to be interviewed if he doesn't want to at this particular stage," Nutter said. "That's kind of how the system works."
Goggin says his client, who will talk with investigators "when they ask," already told them "everything he knew. He cooperated fully."
8th body pulled from the wreckage
The train, which was on time on its trip Washington to New York, was carrying 238 passengers and five crew members when it derailed at about 9:30 p.m. Tuesday. Survivors recalled an otherwise sleepy, mundane ride devolving into chaos as cars tilted and toppled, sending most everything -- from luggage to laptops, from phones to people -- flying.
Eight people died
. They include Associated Press video software architect Jim Gaines, U.S. Naval Academy Midshipman Justin Zemser and Derrick Griffith, a dean of student affairs for City University of New York Medgar Evers College.
The latest fatality is a person pulled from the wreckage of the first car Thursday morning. City Fire Commissioner Derrick Sawyer said a call came in around 8 a.m. "to bring back our cadaver dog," and after that "we were able to find another passenger in the wreckage." That body was taken to the medical examiner's office.
With that discovery, Nutter said all 243 people thought to have been on the train are accounted for. At least six people remain in critical condition. But most of the more than 200 people injured have been or will soon be released from hospitals.
"I want to express my gratitude for the first responders, who raced to save lives, and for the many passengers who, despite their own injuries, made heroic efforts to get fellow passengers to safety," President Barack Obama said Thursday, offering prayers for the victims.
Mayor calls engineer's driving 'reckless,' NTSB fires back
Bostian, the engineer, is among those recovering. As he does, Philadelphia's mayor singled him out.
In a CNN interview Wednesday, Nutter called the engineer's driving "reckless," adding, "There's no way in the world he should have been going that fast into the curve."
"I don't know what was going on with him. I don't know what was going on in the cab, but there's really no excuse that can be offered, literally, unless he had a heart attack."
Sumwalt, the NTSB board member, immediately blasted the mayor's comments as inflammatory.
"You're not going to hear the NTSB making comments like that," he said. "We want to get the facts before we start making judgments."
Nutter referenced his CNN comments during a Thursday afternoon press conference, insisting he has the utmost respect for Sumwalt and the NTSB. He said he'd spoken "from the heart" in the "heat of the moment," expressing an opinion as the mayor and as a citizen.
"In no way, shape or form should my comments be taken as to be judgmental about their process or what might happen or anything else. I often speak from the heart," Nutter said, adding he was not interested in a public back-and-forth with the NTSB. "It is literally meaningless in the whole scheme of things."
Inconvenience for travelers, big hit for Amtrak
For all the high emotions in Philadelphia, things were decidedly calmer at that city's main train station and those in Washington and New York -- because many trains simply weren't running.
New York's Penn Station was relatively sedate for a weekday, as evident by rows of empty seats and Amtrak ticket counters. Local trains ran as normal, and other workarounds were in place -- such as New Jersey Transit honoring Amtrak tickets between New York City and Trenton, New Jersey.
For many travelers, it was an inconvenience. But it's a big deal, and a big loss, for Amtrak.
The entity also known as the National Railroad Passenger Corporation had more than 31 million passengers
between October 2012 and September 2013. Far and away, its Northeast Corridor is its busiest stretch, with more than a third of all passengers.
Thus, Amtrak's three busiest stations -- Penn Station, Philadelphia's 30th Street Station and Washington's Union Station, with annual riderships between 10 and 4 million each -- are also the hardest hit.
Amtrak CEO Joe Boardman said the crash was Amtrak's first in the Northeast Corridor in 28 years, during which time 300 million passengers had ridden its rails. He expects they'll be able to do so again next week, starting with limited service between Philadelphia and New York on Monday.
"We're really looking at full service by Tuesday," Boardman said.
Official: Technology could have prevented crash
The derailment was Amtrak's ninth this year, according to the Federal Railroad Administration
. But it's not just Amtrak. The FRA notes, for example, there were 35 derailments nationwide on main railways in a single month, January 2014.
It happened again Thursday morning, when several cars from a cargo train jumped the track in Pittsburgh's Hazelwood neighborhood, city spokesman Timothy McNulty said. That derailment was relatively minor: McNulty said there were "no passengers, no hazmat, no hazard."
But incidents such as the one in Philadelphia highlight questions about whether the nation's rail infrastructure is adequate.
Transportation analyst Matthew L. Wald said the area where the train derailed Tuesday has had problems, calling it "an extremely heavily used stretch of track." While acknowledging some rails need to be replaced and curves are worn, Boardman told CNN that, overall, the railway there is "in good condition."
In addition to the track's condition, there's a matter of the technology tied to it. Many have noted this stretch did not have an automated speed control system called positive train control that could have overridden human errors and slowed the train down.
In 2008, Congress ordered the nation's railroads to adopt positive train control by December 2015 -- but it's looking increasingly unlikely that the deadline will be met
"We feel that had such a system been installed in this section of track," NTSB board member Sumwalt said, "this accident would not have occurred."
That will happen by year's end, Amtrak's Boardman pledged.
"We're very close to being able to cut it in," Boardman said, noting that such technology already exists elsewhere in the Amtrak system. "... We will complete this."