Investigators released surveillance images and video of the suspect.
Police say the two men got into an argument aboard a Manhattan-bound L train that turned physical.
It happened Wednesday morning at about 8:30 a.m.
Police say the suspect cut the 61-year-old's cheek before taking off on 14th Street.
The victim was taken to Bellevue 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 on Twitter, @NYPDTips. All calls are kept confidential.
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