Police are appealing for witnesses to an indecent assault on an inner-city train last month.

Police said a 33-year-old woman was attacked on a Craigieburn-bound train as it traveled from Flinders Street to Newmarket Station just before 7pm on April 21.

The incident occurred after a Hawthorn vs Geelong AFL game.

Transit Crime Investigation Unit detectives arrested a 30-year-old Maribyrnong man in relation to the incident. He has been released pending summons.

Police believe a number of people were on the train at the time and are appealing for witnesses to come forward. They particularly want to speak to a man who was seen speaking to the woman after the incident.

The alleged offender is believed to have got off the train at Ascot Vale.

Anyone with any information should call Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au