A man exposed himself to a member of rail staff after the Derby County FC and Chesterfield FC match.
British Transport Police (BTP) has released three CCTV images of men they would like to speak to about the incident, which took place on the 4.25pm Plymouth to Leeds service at about 8.50pm on Saturday, January 24.
A man exposed himself to a worker on board the stationary train at Derby station.
PC Christie Ingram, of BTP, said: “A group of four or five men believed to be aged between 19 and 24 were on board the train as it stood at Derby station after the match between Derby County FC and Chesterfield FC.
“One of the men exposed himself to a member of rail staff who was on the train while some of the other men grabbed his underwear and ripped it from him.
“The man’s behaviour was vulgar and offensive and left the staff member and other passengers feeling uncomfortable and intimidated.
“Enquiries have been ongoing and I am now in a position to release CCTV images of men we believe may have key information in relation to this incident.”
PC Ingram added: “The train was full at the time and we would like to hear from anyone who was travelling on the service.”
Contact BTP on 0800 40 50 40 or by text on 61016, quoting reference number B8 MSUB of 26/2/2015.