A woman who managed to take a photo of a man who allegedly sexually assaulted her on a Sydney train was twice targeted in separate incidents, leading to his arrest.

Police say the 20-year-old woman was travelling on a city-bound train in March when a man got on at Westmead, in Sydney's west, and sat next to her.

Police allege he then touched her leg and crotch before he got off the train at Central Railway station.

During the incident the woman managed to take his photograph before she reported the man to police.

About 3.30pm (AEDT) on October 15 the same man got on her train at Central Railway Station and again sat next to her, police say.

He then allegedly indecently assaulted her before she moved to another carriage and contacted police.

Police stopped the train at Parramatta station and subsequently arrested a 46-year-old Glenwood man.

He was charged with assault.

The man was re-arrested on Thursday and charged with additional assault offences.

He was refused bail to appear at Blacktown Local Court on Friday.

Anyone who has been a victim of an attack on a train is urged to contact Crime Stoppers on 1800 333 000.