Authorities say he rubbed the child's thigh while on a southbound A train just before 7 a.m. Monday near the 14th Street Station.
The suspect apparently ran away after the boy's 20-year-old sister yelled at him and alerted the train's conductor.
The suspect is described as 30 to 40 years old with a salt and pepper goatee, last seen wearing a white long-sleeved shirt and blue jeans.
UPDATE: Police have apprehended a suspect in connection with this incident.
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