A man exposed his genitals to a woman he was sat next to on board a Cross Country service at approximately 8.20am on Saturday, September 15.
Detectives investigating the incident believe the man shown in the CCTV image may have information which could help them.
A British Transport Police spokesman said: “Unwanted sexual behaviour on the rail network will not be tolerated and BTP works hard with the rail industry to investigate every offence.
“Our Report It to Stop It campaign, which aims to combat unwanted sexual behaviour on the railway, encourages members of the public to report it, in a bid to build a picture of the offender, which in this case is exactly what happened and aided our investigation.”
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Anyone who recognises the man or has any other information can contact British Transport Police by sending a text to 61016 or call 0800 40 50 40, quoting reference 154 of September 15.
Alternatively ring the independent charity Crimestoppers anonymously on 0800 555 111.