The National Transportation Safety Board did not say what the condition was.
But the NTSB also said Friday that a joint bar, used to hold two sections of rail together, had been cracked and repaired last month. Rail sections in the area of the derailment have been shipped to Washington for further examination.
NTSB says Metro-North railroad is conducting an inspection and inventory of all the joint bars on its main tracks.
The train derailed May 17 in Bridgeport and collided with an oncoming train, injuring more than 70 and disrupting service for days on the railroad used by tens of thousands of commuters north of New York City.