A woman has been charged with assault after she allegedly racially abused and punched a teenage girl on a Sydney train.

The 34-year-old woman was arrested at Mount Druitt police station on Wednesday and issued with a future court attendance notice for the offence of common assault, police said.

Police will allege in court that the woman, from Bidwill in Sydney's west, verbally and racially abused the 15-year-old girl, who is of Asian appearance, before punching her several times in the face and pulling her hair on a train near Lidcombe on Monday afternoon.

The teenager told police she had boarded the train at Blacktown about 2pm with two friends. Her alleged attacker boarded the same carriage at Parramatta.

The woman, who was travelling with a man, sat down on the seat in front of them.