Jim Southworth

Jim Southworth, the NTSB investigator in charge of the probe into the Ellicott City derailment, holds a news conference on Main Street. (Baltimore Sun photo by Kim Hairston / August 21, 2012)

Within hours of the CSX train derailment that killed two young women in Ellicott City early Tuesday, investigators from the National Transportation Safety board were on the scene to determine why it happened.

When tragedy strikes in the nation's skies or on its roads, rails and waterways, the NTSB is where the nation looks for an explanation of what went wrong, but answers seldom come quickly.

It could take months or even years before the independent federal agency reaches a final conclusion on the cause of the train derailment that ended with Rose Mayr and Elizabeth Nass, both 19, buried in coal on a railroad bridge in the historic downtown.

Jim Southworth, the NTSB's lead investigator, said the agency has not determined the cause of the accident. He said its experts had just begun the painstaking process of examining every aspect of the incident.

"It is a slow process, but it is very productive," he said.

Experts interviewed by The Baltimore Sun also were reluctant to speculate about what might have caused the Ellicott City derailment based on news reports. Derailments can be caused by problems with the tracks or rail bed, excessive speed or defective equipment.

James Remines, a former NTSB investigator who retired two years ago and now lives in Gambrills, said Southworth's statement that the train operator had not applied the air brake but that the air line had ruptured indicated the emergency brake activated only after the derailment had started.

Remines said that if the engineer did not apply the air brake, that would be "a clear indication the victims had no role" in causing the derailment.

"They just happened to be in the wrong place," he said.

John Samuels, a retired senior vice president for operations at Norfolk Southern, said the women were probably "victims of circumstance."

Samuels said a broken air brake line will trigger emergency braking that can cause a derailment. If the derailment comes first, he said, the cars typically fold over one another like an accordion, which separates the air lines, automatically activating the brakes.

The NTSB has not reached a conclusion about whether the women's presence contributed to the derailment, Southworth said. Like all other aspects of the crash, he said, that is still being investigated.

The NTSB seeks to produce not just answers to the questions of how and why a calamity occurred and who was responsible, its mission also is to determine what lessons can be learned to help prevent similar accidents in the future.

Over time, those answers have led to numerous safety improvements in aviation and on railroads, intercity buses, transit lines, ferryboats and other modes of transportation.

In the Ellicott City case, the agency is expected to examine many possible causes — or contributing factors — to a derailment that also blocked traffic and disrupted commerce in the Howard County seat.

Among the factors NTSB investigators will look at are the track conditions along the Old Main Line — one of the oldest railroad rights of way in the United States.

"We will do an exhaustive examination of track maintenance," Southworth said.

Investigators will take the torn-up track apart and piece it together on a flat surface, he said.

Samuels said teams from the agency and the railroad will work their way miles up the track from the scene of the derailment.

"They look for any defects, any indication that the rail has been pocked by something that has been broken," he said.