Investigators say the suspect followed an 18-year-old woman onto a southbound 2 train at 125th Street in Harlem.
As the train approached the 72nd Street subway station, police say the man exposed himself and tried to pull down the woman's pants.
A good Samaritan stepped in and helped the woman get away.
The suspect got off the train at 42nd Street.
Anyone with 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).
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