Transit Divisional Response Unit is appealing for witnesses after a train seat was set alight while the train was stopped at the Laverton Railway Station in March.
Investigators have been told a man boarded the carriage on platform three and poured accelerant onto the seat, setting it alight about 11.10am on Tuesday 16 March.
The man then boarded the next Werribee bound train and got off this service at Williams Landing.
The fire caused damage to the seat and floor of the train which had to be removed from service to be repaired.
No one was injured during the incident.
Investigators have released CCTV and an image of a man they believe may be able to assist them with their enquiries.
The man is perceived to be Caucasian in appearance, about 30-years-old, 175cm tall and was wearing a dark cap and a grey hoody.
Investigators are appealing to anyone who may have in the vicinity of the Laverton Railway Station about 11.10am on 16 March or anyone who may recognise the man to come forward.