The girl boarded the northbound train at the 125th Street station, near Lexington Avenue, about 6 p.m. on March 26, when the suspect, who was already on board, started grabbing her until she fled at the 161st Street-Yankee Stadium stop, police said.
She was not physically injured in the attack, police said.
Investigators released a surveillance photo of the suspect, who was wearing a large black coat and carrying a duffel bag. He's believed to be between 20 and 40-years-old, police said.
About five days after the 4 train attack, a woman fended off a man who was trying to sexually assault her inside Manhattan's Fulton Street station by biting the hand he'd put over her mouth, police said.
Both attackers remained at large Wednesday morning, a police spokesman said.
Anyone with information is asked to call Crime Stoppers at (800) 577-TIPS (8477).