A woman accused of racially abusing a teenage girl and repeatedly punching her in the face on a Sydney train may have been captured on CCTV camera carrying out the attack, police say.

The 15-year-old girl told police she was simply sitting on the train with two friends when the woman, who was aged about 40 and who had a black front tooth, targeted her as the train travelled between Parramatta and Lidcombe railway stations on Monday afternoon.

Inspector Sean Gabin, from the Flemington local area command, said the teenager and her friends had boarded the train at Blacktown about 2pm and were travelling towards the city, where they planned to meet up with other friends.

The train stopped at Parramatta and the teen's alleged attacker boarded the train with a man and sat down in the seat in front of her.

Inspector Gabin said the girl, who was of Asian appearance, was talking to her friends when the assault began.