"Based on what we know right now," Sumwalt said, "we feel that had such a system been installed in this section of track, this accident would not have occurred."
Here are some answers to key questions about the technology, which transportation safety advocates have been pushing for years:
It's a system that combines GPS, wireless radio and computers to monitor trains
and stop them from colliding, derailing or speeding.
If a train isn't being operated in accordance with signals, speed limits or other rules, the system will slow or stop it.
What does it look like?
Former NTSB member John Goglia said the devices he worked with in the 1990s were relatively small black boxes, 6 inches wide, about 8 inches tall and about 15 inches long.
"I'm sure today they're even smaller," he said.
The boxes are often installed in the cab near the front of the engine. "But it can be anywhere," he said.
Installation can take relatively little time, perhaps a few days.
As an NTSB board member, Goglia worked for years with the Federal Railroad Administration to develop the technology.
Is there an example where it is known to have averted accident?
Since positive train control is not widely used, it's hard to say. But the NTSB has said that positive train control technology could have prevented numerous railroad accidents that involved human error.
The agency has given the example of the December 1, 2013, Metro-North commuter train derailment in the Bronx, New York, in which four people were killed and dozens injured. The engineer had fallen asleep and failed to slow the train from more than 82 mph to the maximum authorized speed of 30 mph as it entered a curve, the NTSB said.
The NTSB has said that since 2004 it has completed investigations of at least 25 train accidents that took 65 lives, injured more than 1,100 people and caused millions of dollars in damages -- all of which could have been prevented or mitigated by positive train control.
What does it cost?
In a 2013 NTSB hearing
, Mark Hartong of the Federal Railroad Administration said it would cost $875 million for initial acquisition and installation on commuter railroads and Amtrak. Additional installation and operating costs are estimated to be from $9.5 billion to $13.2 billion over a 20-year period.
The Southern California Regional Rail Authority (Metrolink) -- which operates seven routes with 169 weekday commuter trains on more than 430 route miles, and carries more than 43,000 riders a day -- has started implementing positive train control on three of its seven lines at a cost of $216 million.
"The real dilemma that you face ... is you have a limited, a finite amount of money each year that you can spend on infrastructure and safety," said Peter Goelz, a former managing director of the NTSB. "Do you spend the money on high consequence, low probability events? And I would call the accident two days ago high consequence but low probability. Or do you spend it on high probability, lower consequence items such as working on electrical systems or air conditioning?"
How many rail systems have it?
According to the Federal Railroad Administration, safety systems have been implemented or are being developed and tested since 2007 on the following rail lines:
CSX Transportation on 138.6 miles of track from Spartanburg, South Carolina, to Augusta, Georgia; CSX Transportation on 110 miles of track from Spartanburg, South Carolina, to Erwin, Tennessee; the Burlington Northern Santa Fe Railway (BNSF) on 130 miles from Beardstown to Centralia, Illinois; BNSF on 115 miles of track from Fort Worth, Texas, to Oklahoma City, Oklahoma; Amtrak on 66 miles from Niles to Kalamazoo, Michigan; the Transportation Technology Center Inc. test track in Pueblo, Colorado; Alaska Railroad on 350 miles of track from Anchorage to Fairbanks; Amtrak on 198 miles from Boston, Massachusetts, to New Haven, Connecticut; Amtrak on 75 miles of track from New Haven, Connecticut, to New York City; Norfolk Southern on 110 miles from Columbia to Charleston, South Carolina; Union Pacific Railroad (UPRR) on 193 miles from North Platte to Morrill South, Nebraska; UPRR on 140 miles from Spokane, Washington, to Eastport, Idaho; New Jersey Transit on 21 miles of track from Spring Valley, New York, to Woodridge, New Jersey; New Jersey Transit on 60 miles of track from Port Jervis, New York, to Secaucus, New Jersey.
Metrolink expects to have PTC technology throughout the 216-mile publicly owned portion of its 512-mile rail network before the end of 2015.
What are the benefits of the system?
Positive train control was designed to prevent the human errors behind roughly 40% of train accidents, rail safety experts say.
Imagine what would happen, the NTSB said in a fact sheet this year
, if an engineer suffering from a cold didn't notice a red signal and failed to stop the train.
"With PTC, the train stops anyway," the NTSB said. "Without PTC, real world results have been tragic. ... Without it, everybody on a train is one human error away from an accident."
Why wasn't it installed on this stretch of track?
"That's going to be a key question. We want to find out," Sumwalt told CNN's "The Situation Room." "Why was it in other areas ... on Amtrak's Northeast Corridor, but why was it not here?"
A system that can stop accidents sounds like a no-brainer. But there's a catch: It's expensive.
That's one reason the railroad industry has opposed it in the past. And that's likely one reason why even though it has been installed in most of the Northeast Corridor for Amtrak, it hadn't yet been installed on the part of the track where the train derailed Tuesday night, Goelz said.
"It costs billions," Goelz said. But that doesn't mean it isn't worth the price, he said. "We cannot stop. The government needs to help."
Goelz also offered another possible reason the system wasn't in place.
"It could be because this is an intersection of a number of different rail lines, at this intersection they did not have it in place even though it was a relatively sharp turn," he said.
Are there plans to use positive train control more in the U.S.?
Amtrak CEO Joe Boardman said he is "committed to meeting the requirement of positive train control" technology that a NTSB official said could have prevented the deadly crash in Philadelphia. "That will happen on the Northeast Corridor by the end of this year," Boardman said.
In response to a head-on collision that killed 25 people in 2008 near Los Angeles, 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. And some lawmakers have proposed extending it to as late as 2020.
Amtrak said in a newsletter this year
that it had plans to implement positive train control on 1,200 more miles of track, including the remainder of its Northeast Corridor.
At this point, most freight railroads said they won't be able to make the December 2015 deadline, Sumwalt told CNN. But it's something the safety board is still pushing for, especially in light of this week's derailment.
"We feel that it needs to be implemented," he said, "because it will prevent the very type of an accident that we're talking about here."