The piece of rail that cracked in Queens, causing a subway train to derail, replaced another piece where a crack was found just a couple of months ago. NY1's Jose Martinez filed the following report.
So what caused a new piece of rail to give way, leading to the derailment that injured 19 F train riders on Friday morning?
"A rare event like a subway derailment with passengers involved needs a swift answer," said Gene Russianoff of Straphangers Campaign.
Especially since the rail that broke was installed less than two months ago to replace another section at the same spot that had been found to have a hairline fracture.
"We're going to send this piece of rail out to scientists, who can take a look at it under a microscope, analyze the metal involved and get us an exact answer of how this rail broke and why," said Metropolitan Transportation Authority spokesperson Adam Lisberg.
The MTA said it's also looking into whether there may a link between the two breaks. The rail that snapped had been in place for just seven weeks after being manufactured in the United States last November, so the MTA said age was clearly not a factor in whatever caused it to break.
Officials said it was manufactured by ArcelorMittal, a company that's supplied rail to the MTA for decades, and which bills itself as "the world's leading steel and mining company."
In a statement, the MTA said all rail is rigorously inspected before it's used.
"The manufacturer conducts its own quality assurance process, with ultrasonic testing of every rail and laboratory tests of the steel composition. MTA inspectors review those test results and conduct additional tests on samples of the rails," the statement read.
The agency said inspectors walk the length of the system twice a week in search of defects. Specialized rail cars also search for potential trouble every two or three months, which is how they found the hairline crack in the earlier piece of rail.
Transit officials said the Queens Boulevard line had 205 rail breaks between 2005 and 2012, none of them leading to derailments. It's targeted for rail improvements in the agency's 2015 to 2019 capital plan.
"That's going to make a big difference in terms of having a smoother ride, and it's going to be a way to address the areas with the most broken rails first," Lisberg said.
First, though, some big questions need answers.