It happened Sunday around 1 p.m. on the northbound R train.
Investigators believe the suspect approached a 64-year-old man and repeatedly punched him in the face, causing facial fractures and breaking his nose.
Police say the suspect fled the train at the Prospect Avenue station, and police are still searching for him.
The victim was taken to the hospital in stable condition.
Anyone with any information is asked to call the NYPD's Crime Stoppers hotline at 1-800-577-TIPS (8477), or for Spanish, 1-888-57-PISTA (74782). You can also submit a tip via their website or via DM on Twitter, @NYPDTips. All calls are kept confidential.
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