Police say Detective James Griffin, 41, was trying to get an emotionally-disturbed man off of the subway tracks Thursday morning in Times Square.
The man fought with the officer, and Griffin fell and hit his head on one of the rails.
Other officers rescued him and took him to the hospital.
The suspect, identified as 29-year-old Ricard Wilcox, is facing charges.
The MTA had shut down service on the N, Q and R trains after receiving reports of a man on the tracks.
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