The rail that broke underneath a train in Queens on Friday morning was manufactured in the U.S. in November and installed in March. The other rails from that shipment will be tracked down and inspected, according to the MTA.
The cause of the derailment, the most significant in years on the nation's largest subway system, remains under investigation.
The express F train was heading for Manhattan and Brooklyn when six of its eight cars derailed around 10:30 a.m. more than 1,000 feet from the nearest station. The train was plunged into darkness, and sparks and smoke partially filled some of the cars.
Four people suffered serious injuries and were hospitalized after the derailment, which took place about 30 feet below street level. Fifteen other straphangers, some of whom complained of chest pains, were treated at the scene.
Dozens of emergency workers converged quickly on the scene, some of whom used ladders to help passengers descend from the train to track level and guided them along the track to an opening in the sidewalk. Power was cut to the third rail to aid the rescue.
MTA workers are repairing a damaged 500-foot stretch of rail and aim to have local service restored by Saturday afternoon. Officials hope to restore full service by Monday morning's rush hour.
The area where the incident occurred is one of the five zones the MTA has identified as "critical rail break" corridors that need replacement, according to The New York Daily News. However, because the age of the rail does not appear to be a factor, MTA investigators will focus on other areas while trying to determine what caused the derailment.
There is no signal switch in the area. The train's operator and conductor were tested for drugs and alcohol, but results were not immediately known.