[UPDATE BELOW] It may take weeks or even months for investigators to definitively determine the cause of the deadly Metro-North train derailment in the Bronx on Sunday morning. But speed was almost certainly a factor in the crash, which occurred at a well-known curve in the tracks where train operators are required to drop from high speeds of 70 mph down to 30. Why the train was traveling too quickly for the sharp turn remains to be seen. The operator, William Rockefeller, said the brakes weren't working, and so he took the emergency measure to "dump the brakes." The Times explains what that means:

Dumping brakes is a last-resort move typically reserved for averting collisions with other trains or cars stuck at crossings, said Grady C. Cothen, a retired federal railroad regulator. He said it involved slamming the emergency brakes on the wheels on all cars at once. To achieve that, the operator quickly releases, or “dumps,” all air from the train’s brake lines, he said. “If he did that, then it would have been an act of considerable alarm,” Mr. Cothen said.

Rockefeller reportedly told investigators that the brakes didn't work when he neared the curve. (Union officials say Rockefeller has a flawless record.) "No one can figure out, really, what happened to the engineer, but he did not brake the train," a source familiar with the investigation told the Wall Street Journal. The dramatic bend is familiar to train operators, but nevertheless, a 24-car freight train carrying garbage south of the Spuyten Duyvil station derailed last July, and of course there was that famous derailment at the same location in 1882.

A federally mandated rail safety system called Positive Train Control, or PTC, is supposed to be implemented nationwide by 2015. The sophisticated computer system would automatically force trains to slow down when entering sharp curves. It's too soon to tell if PTC could have prevented Sunday's derailment, which killed four passengers and injured 63, but Metro-North has already told federal regulators that it won't meet the 2015 deadline.

Update 4:28 p.m.: At a hearing today, a National Transportation Safety Board member said information from the two "event recorders" reveal that the train was going 82 miles per hour as it entered the 30-mph curve.