Police want to talk to the man in the picture about the incident on the service from Euston to Birmingham on November 7.
While the train was between Leighton Buzzard and Milton Keynes at around 7.20pm, a woman noticed a man staring at her.
While staring, she says he was touching himself over his trousers.
Officers think the man in the picture may have information that could help with their investigation into the incident.
A British Transport Police spokesperson said: “Anyone who recognises him is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40, quoting reference 702 of 7 November.
“You can also contact Crimestoppers anonymously on 0800 555 111.”
