Around 30 people are understood to have been fighting in the fourth carriage of the 9.58pm service from Glasgow Central to Larkhall on Saturday.
Police received reports of the incident at around 10.10pm, ten minutes after the scuffle broke out.
They are now reviewing CCTV images in a bid to trace those involved and are asking the public for their help.
Detective Constable Kevin Foulis, the BTP officer in charge of the enquiry, said: "Anyone who was on the train or at the station and can provide information which can help the enquiry is urged to contact BTP."
Anyone with information is asked to contact British Transport Police on 0800 40 50 40 or by texting 61016.
Anonymous information can be given through Crimestoppers on 0800 555 111.