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By Tom Avril, The Philadelphia Inquirer (TNS)

PHILADELPHIA — When you get right down to it, it was a question of physics.

At the site of the Amtrak derailment on Tuesday, the track had a curvature of 4 degrees. Imagine a giant circle with a diameter of nearly 2,900 feet, more than a half-mile. The track’s path would trace the outline of that circle.

The track also had a “superelevation” of 5 inches, meaning the outer rail was5 inches higher than the inner rail.

Given those parameters, a locomotive pulling seven Amtrak-size cars could safely travel up to about 55 mph, said Pennsylvania State University engineer Steve Dillen, who performed a rough calculation at The Philadelphia Inquirer’s request.

Early indications are that it was going nearly twice that fast, a National Transportation Safety Board member said Wednesday at a news conference.

Exactly how much faster will have to wait for detailed data from an onboard “event recorder,” analogous to what is known on airplanes as a black box. On trains, these devices electronically track speed, direction, distance, throttle position, and the use of brakes, among other data.

“It’ll tell them everything that train did,” said Danny Gilbert, a rail safety consultant based in Roanoke, Va.

Such recorders have been required on trains for years, but data sometimes have been lost in violent accidents. In 2005, the Federal Railroad Administration enacted rules requiring the devices to be crash-resistant.

Speed is typically measured using an axle-mounted generator that pulses with every revolution. Such generators often are mounted on more than one axle; their outputs can then be compared to correct any issues with slipping and sliding. The data is then transmitted to the event recorder.

It was not immediately known who made the box or boxes on the Amtrak train that derailed. Mayor Michael Nutter said they had been taken to an Amtrak facility in Delaware for analysis.

The section of track where the train derailed had a 50 mph speed limit.

Dillen, who coordinates the rail transportation engineering program at Penn State Altoona, said an additional 5 mph would probably be OK. Beyond that, he could not say.

Determining the exact tipping point, he said, would require a more detailed analysis involving the train’s center of gravity.

(c)2015 The Philadelphia Inquirer, Distributed by Tribune Content Agency, LLC.

Multiple injuries are reported during an Amtrak crash of a northbound train in Port Richmond on Tuesday, May 12, 2015, in Philadelphia. (Tom Gralish/Philadelphia Inquirer/TNS)

Jeff Danziger lives in New York City. He is represented by CWS Syndicate and the Washington Post Writers Group. He is the recipient of the Herblock Prize and the Thomas Nast (Landau) Prize. He served in the US Army in Vietnam and was awarded the Bronze Star and the Air Medal. He has published eleven books of cartoons and one novel. Visit him at DanzigerCartoons.