The man threatened the ticket collector when he came up to check that he had paid his fare, and demanded money. There was then a brief struggle.
The train worker was not injured but was left shaken by the incident which happened at around 7.50pm on Friday on the the 7.06pm Larkhall to Dalmuir service.
British Transport Police (BTP) are reviewing CCTV footage to try to identify the man and appealed for information.
Detective Constable Craig Jennings, of BTP, said: "I am aware there were other passengers on the train and indeed a man telephoned for police assistance.
"I am asking anyone who was on the train, particularly the man who phoned, and can provide information that can help our inquiry to get in touch."