If Amtrak Train 188 had been heading to Philadelphia from New York City, it would not have derailed at the sharp Frankford Junction curve, because an automatic braking mechanism has been in place for years on the southbound side of the tracks to stop a speeding train.
But Amtrak never installed the same electronics on the northbound side, so Train 188 was able to enter the curve where the speed limit is 50 m.p.h. at more than 100 m.p.h.
Amtrak CEO Joseph Boardman said in an interview that the lack of the automatic-braking control on one side of the curve was "a loophole" that he was unaware of until Train 188 derailed Tuesday, killing eight people and injuring about 200.
The death toll rose by one Thursday, when the eighth victim was found in the wreckage of the mangled first passenger car.
Mayor Nutter said officials now believe all 243 people who were on the train have been accounted for.
The eight victims have been identified through various sources as Jim Gaines, 48, an Associated Press employee, of Plainsboro, N.J.; Abid Gilani, 55, a bank executive, of Rockville, Md.; Bob Gildersleeve, 45, an Ecolab executive, of Elkridge, Md.; Giuseppe Piras, 41, an olive-oil and wine merchant, of Italy; Justin Zemser, 20, a U.S. Naval Academy midshipman; and three passengers from New York City - Laura Finamore, 47, a corporate real estate officer; Derrick Griffith, 42, a dean at Medgar Evers College; and Rachel Jacobs, 39, an online start-up executive.
The closure at Franklin Junction has not only halted Amtrak service between New York and Philadelphia and, by extension, New York and Washington, it also has forced 12,000 daily riders who use SEPTA's Trenton Line to find alternate routes to work, and has prevented NJ Transit's Atlantic City Line trains from getting to and from 30th Street Station. About 1,200 riders use that service daily.
Boardman said limited-to-full service was expected by Monday morning.
He said that the railroad had not installed automatic-braking technology on the northbound side of the curve under the assumption that trains just leaving 30th Street Station would be slower than those barreling south from New York.
That component of Amtrak's decades-old "automatic train control" system is less sophisticated than a new "positive train control" system required by federal law to be installed on all passenger and major freight railroads by the end of 2015.
Boardman defended Amtrak's decision not to install positive train control yet at the curve. Such a system would have automatically braked the train and prevented the derailment, federal investigators have said.
He said Amtrak needed to get additional radio-transmitting capability for the system to work in the curve. Now, the railroad is aggressively installing the new train control system along the Northeast Corridor and will meet a federal deadline of Dec. 31 for installation, he said.
He said Amtrak still planned to install PTC at the curve this year, at the same time as the rest of the corridor between New York and Washington.
"We're the leader in PTC," Boardman said, adding that "with more financing, we could have done it sooner."
The positive train control system would have prevented Tuesday's derailment, National Transportation Safety Board member Robert Sumwalt said this week.
So would the existing system, if it had been installed on the northbound side, railroad engineers say.
Boardman said he and other top Amtrak management were unaware the speed-control mechanism was not in place on the northbound tracks before Tuesday night's crash.
"I didn't know that particular condition existed until this occurred," Boardman said. "We probably would have changed it, but we didn't know about it."
Engineers for Amtrak and other railroads operating on the Northeast Corridor, the nation's busiest rail route, were well aware of the anomaly.
A veteran SEPTA engineer who frequently operates trains on the corridor said engineers in southbound trains have six seconds to start braking after they are alerted by both lights and sounds in the train cab if they are going too fast. The engineer asked not to be identified.
If the engineer does not slow to 45 m.p.h., the train is automatically stopped, he said. No such alerts or automatic braking occur for northbound trains approaching the curve.
For northbound trains, the speed limit for Amtrak trains leaving Philadelphia rises from 60 to 65 to 80 m.p.h. before the Frankford curve, where the limit drops to 50.
For southbound trains, the speed limit before the curve is 110 m.p.h.
Amtrak's Boardman said trains could negotiate the curve at 80 m.p.h., so planners apparently decided it was unnecessary to install the "code change" that would have triggered emergency braking if a northbound train did not slow down from the previous speed limit.
"The theory was to put it where it would result in an accident if you didn't have it," Boardman said. "You can get through it at 80. You could make it, though it would have been a rough ride."
Train 188 was pulled by a powerful new Siemens "Cities Sprinter" locomotive that Amtrak put into service last year to replace locomotives that were 25 to 35 years old.
The 8,600-horsepower Siemens locomotives can accelerate more quickly than their Amtrak predecessors, which were operating when the train-control systems were installed.
Brandon Bostian, the engineer of the derailed train, would not have received beeping or flashing alarms approaching the Frankford curve at high speed from the south, since an electronic transponder to communicate with the train had not been placed in the tracks on the northbound side.
"There's no enforcement on speed there," one engineer said of the northbound side. "But we're all supposed to do our jobs and know what we're doing. We all know about the accident in the '40s there."
(On Sept. 6, 1943, the Pennsylvania Railroad's Congressional Limited crashed at the Frankford curve, killing 79 people and injuring 117.)
He said it was "astonishing" that an engineer would approach the Frankford curve at such a high speed.
Rep. Robert Brady (D., Pa.) called for the Federal Railroad Administration to publicly release all documents relating to the implementation of positive train control technology mandated by Congress in 2008.
"This week's tragedy in Philadelphia was not caused by a failure of technology; it was a failure of political will to implement the technology we have that could have avoided the loss of lives," Brady said in a statement.
Boardman also defended Amtrak's safety record, even as he lamented that Tuesday's derailment may "have destroyed the confidence of people" who ride the railroad.
He also said Amtrak has been underfunded for decades, and needs more money from Congress to rebuild the century-old underpinnings of the Northeast Corridor.
Meanwhile, Temple University Hospital, where the most seriously injured were taken, reported that after starting the day with 16 accident patients, the number had dropped to 11 by Thursday afternoon, six of whom were in critical condition, two fewer than in the morning.
Chief medical officer Herbert Cushing said all those in critical condition were expected to survive.
Inquirer staff writers Mike Newall, Caitlin McCabe, and Laura McCrystal contributed to this article.