A medical document made available Monday by the National Transportation Safety Board says William Rockefeller has "severe obstructive sleep apnea."
The NTSB did not say whether the engineer's disorder contributed to the crash.
Apnea can disrupt normal sleep and result in sleepiness. Rockefeller's lawyer and union leader have suggested the engineer nodded off as the train raced toward a sharp curve in the Bronx, where it derailed.
The document says Rockefeller had not been tested for the disorder before the Dec. 1 derailment.
The Metro-North railroad said it was reviewing the document.
The NTSB said its determination of the cause would come in its final report.