The British Transport Police today said the man boarded the 9.40am Northern service from Leeds to Sheffield on March 10.
As the train travelled to Wakefield Kirkgate, the man was challenged by the guard as he did not have a ticket, officers said.
The man then assaulted the member of staff and verbally threatened members of the public, the force said.
The guard was not injured as a result of the assault.
At Wakefield Kirkgate station, the man left the train and ran over the tracks to leave the station.
The incident happened between 9.40am and 10.05am.
Officers would like to speak with the man shown in this image as he may have information which could help the investigation.
Call BTP on 0800 40 50 40 or text 61016, quoting reference number 117 of 10/03/18.
Alternatively, pass information anonymously to Crimestoppers on 0800 555 111.