The incident happened on Thursday, 24 July, on board the 7.23am Colne to Blackpool South service as it travelled between Burnley Central and Burnley Rose Grove.
PC Chris Haughton said: â€œA man boarded the train at Burnley Central and became verbally abusive when the train guard, a 47-year-old Asian man from Blackpool, asked to see his ticket.
â€œDuring this abuse the man also threatened the guard, telling him he would knock him out.â€
Following the incident the offender left the train at Rose Grove.
PC Haughton added: â€œThe train guard was simply trying to do his job and certainly should not have been subjected to any form of abuse or threats of violence.
â€œCrime on the rail network, particularly violence, is rare and BTP is committed to further reducing crime and protecting rail staff who provide a public service.â€
The aggressive man is described as being white, in his 60s, about 5â€™ 8â€ of skinny build with untidy hair and unkempt facial hair. At the time of the incident he was wearing a long sleeved grey, checked, knitted jumper with a collar and dirty, scruffy jeans.
PC Haughton said: â€œIf you recognise the man in the e-fit, or have any information about the incident please come forward.â€
If you can help, please call British Transport Police on 0800 40 50 40 or ring Crimestoppers, anonymously, on 0800 555 111. In all calls please quote log PSUB/B10 of 14/10/14.