British Transport Police (BTP) said the attack took place on a link between Airdrie and Balloch at around 1.40pm on Wednesday 1 March.
The woman was approached by two men on the train and sexually harassed by one of them.
He got off the train at Singer station in Clydebank, West Dunbartonshire.
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He is described as white, in his late 30s or early 40s, and spoke with a foreign accent.
Anyone with information is asked to contact BTP on 0800 40 50 40 or by texting 61016 with reference 2300023645.
Tips can also be given anonymously to the independent aid organization Crimestoppers on 0800 555 111.
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