A train guard was threatened and spat on a train service between Keighley and Cross Flatts.
British Transport Police are now investigating the incident, which took place on Monday, January 25, at around 2.50pm.
Investigating officer PC Sam Bloomfield said: “The train guard, a 56-year-old man from Keighley, had approached a passenger asking to see his ticket.
“The man said he had torn up his ticket at Keighley and was travelling to Bingley, but refused to buy another.
“When the guard asked the man to leave the train at Cross Flatts station and he became aggressive and began threatening the guard.
“He then spat twice at the guard and threatened to head butt him.”
Officers have been pursuing several lines of enquiry, including analysing DNA from a swab of saliva found on the victims clothing.
They now want to speak to the man in the image in connection with this incident.
PC Bloomfield added: ““This was a vile attack on a member of staff, who was simply doing his job. All rail staff have the right to come to work without the fear of violence or intimidation of any kind. We treat all reports of staff assault very seriously and will always thoroughly investigate.”
If you know the man pictured, or have any information about the incident, please give us a call on 0800 40 50 40 or text 61016 quoting reference PSUB/166 of 5/2. Alternatively, information can be passed to the independent charity Crimestoppers on 0800 555 111.