At just before 4.30pm on Tuesday 26 October, the victim was approached by a man and a woman. The man then threatened him and brandished a weapon before forcefully taking his bike.
The incident happened as the train travelled between Bellshill and Carluke.
The man is described as being of slim build, around six foot and between 25 and 30 years old. His hair is described as dirty blonde, short on the sides and curly on the top, and on the day of the incident he was wearing a black jumper.
Officers are keen to speak to anyone who may have witnessed the incident to assist their enquiries.
Witnesses, or anyone with information, can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 385 of 26/10/21.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
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