By Ted Mann 

A safety system to stop speeding trains had been installed on the southbound tracks near the derailment in Philadelphia that killed eight people, but not on the northbound side, where the Amtrak train hurtled off a sharp curve at over 100 miles per hour.

People familiar with Amtrak's signal system said speed-control measures used elsewhere by the railroad could have prevented Tuesday's crash, which also injured more than 200.

Amtrak officials said they were concerned enough about the curve to install the added protection on the two southbound tracks. But it wasn't installed on the northbound side because officials didn't believe the tracks leading to the curve would allow trains to build up enough speed to topple over.

Federal investigators said this week a different, far more expensive and technically advanced system--called positive train control--would have prevented the crash. Amtrak said it will have positive train control installed along the entire Northeast Corridor from Washington to Boston by year's end.

But safety experts and officials of other railroads said that Amtrak's existing system--called automatic train control and used for decades by many railroads to keep trains safe distances apart--could have been modified to prevent such a crash until the new system is ready.

Amtrak's Northeast Regional Train 188 was traveling at 106 mph as it headed into the tight curve with a speed limit of 50 mph and jumped the tracks, the National Transportation Safety Board determined.

Data analyzed by investigators shows the train steadily accelerated for more than a minute on the approach to the curve, National Transportation Safety Board member Robert Sumwalt said Thursday. Its speed climbed from more than 70 mph to more than 100 mph in 65 seconds, with the engineer applying the emergency brakes just before the recording ended, Mr. Sumwalt said.

Automatic train control systems can be programmed to send information to a train about the speed limit for a section of track. Equipment inside the locomotive senses when a train is exceeding the limit and sets off an alarm. If the engineer fails to slow the train, the system triggers the train's emergency brakes.

D.J. Stadtler, an Amtrak executive vice president, said in an interview Thursday that after a 1990 crash at Back Bay station in Boston, Amtrak decided to install the speed-enforcement measures at a select number of curves based on the risk of a train toppling over. The risk is based in part on the speed limit in the track section before a curve.

Where the maximum speed in the section just before a curve would be an "overturning speed" if it were maintained in the curve itself, the speed-enforcement measure was installed. But installing such protections in areas of lesser risk of a train rolling over--like the northbound tracks where Tuesday's crash happened--would unnecessarily slow Amtrak trains in some places beyond what safety requires, Mr. Stadtler said.

Joseph Boardman, Amtrak's president and chief executive officer, said in a separate interview that the reason a track speed-limiting circuit was in place for southbound trains was "because you were transitioning from 110 miles an hour [the nearby speed limit] to a 50-mile-an-hour curve."

In the case of the northbound train Tuesday, "that train would have gone around the curve at 80 [the limit for the preceding track section]. It would not have come off," he said, explaining why there is no speed-limiting circuit in that direction.

Asked if it might be worthwhile to add track speed-limiting circuits between now and when Amtrak's positive train control system is set to be completed at year's end, Mr. Boardman said, "I don't know the answer to that. I don't know what the cost is, the complexity, the engineering to do something like that."

The electric locomotive hauling the derailed train was one of a new fleet of 70 introduced into service starting last year. Amtrak has said the new locomotives, from Siemens AG, are more powerful than the more-than-25-year-old units they are replacing.

The "mechanicals of the locomotive," such as its acceleration, are "one of the things that NTSB wants to look at," Mr. Boardman said.

Still, Mr. Boardman maintained that Amtrak is "the safest railroad you can imagine," and that "it's been 28 years since there's been a derailment on the Northeast Corridor with this kind of loss."

President Barack Obama said Thursday that the crash underscores the need for the U.S. to invest in updating its infrastructure.

The accident's death toll rose to eight Thursday after another body was pulled from the wreckage, officials said. Philadelphia Mayor Michael Nutter said all 243 passengers and crew had now been accounted for.

Two more victims were identified Thursday. A spokesman for the family of 47-year-old Laura Finamore, a managing director with the real-estate firm Cushman & Wakefield, said she was killed while returning home to Manhattan from a memorial service for a friend's mother. And 45-year-old Maryland resident Bob Gildersleeve, who had been reported missing, was confirmed dead, according to Ecolab, where he worked as a vice president.

Pat Reilly, a former district chief for the Federal Railroad Administration, said he and a colleague asked supervisors in the late 1980s to require speed protections on sharp curves along Amtrak tracks.

But he said his request went nowhere until after the Boston crash, in which a federal probe found that an apprentice Amtrak engineer had entered a curve at more than twice the speed limit. The train derailed and collided with a commuter train, injuring hundreds of passengers.

An NTSB investigation found that Amtrak's failure to "have advanced warning devices for a speed reduction for the curve" was a factor in the crash.

Railroad signals tell engineers when they can safely proceed. "Cab signaling" systems can be equipped to pick up electrical signals that are pulsed through sections of rail, and which transmit speed limits to a display in the locomotive's cab.

For railroad executives, Tuesday proved an eerie echo of the 2013 New York City crash on Metro-North Railroad, when an engineer dozed off, according to federal investigators, before speeding into a curve and derailing. Four passengers of the commuter line died.

In that case, Metro-North also had relied on the skill and training of engineers, who are required to know every mile of their rail territory, and to use their own judgment in many instances when slowing down for hazards such as sharp curves.

Metro-North didn't have a speed-limit circuit ahead of the curve at Spuyten Duyvil, N.Y., but quickly installed one, at the behest of the FRA, after the crash.

"On the surface, this accident seems very similar to the one that occurred at Spuyten Duyvil," said Jeffrey D. Knueppel, deputy general manager at the Southeastern Pennsylvania Transportation Authority, the Philadelphia-area transit agency.

Write to Ted Mann at ted.mann@wsj.com

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