On 11 March, a woman was travelling on the 1,53pm service when she was approached by a man who began speaking to her inappropriately before sexually assaulting her.
He is described as black, around six-foot, medium build, with short black hair and a full black beard. He was wearing a green jacket and carrying a rucksack.
The man left the train at Glasgow Central.
The train was busy at the time of the incident and detectives are appealing for anyone who witnessed what happened or has any information to come forward and assist the investigation.
Anyone with information is asked to contact BTP by calling 0800 40 50 40 or by texting 61016 – with reference 2300034876.
Information can also be given anonymously to the independent charity Crimestoppers on 0800 555 111.
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