Police want to trace a man who stripped naked and ran through a crowded train carriage.
The streak happened between York and Middlesbrough on Friday at around 8.30pm.
British Transport Police have released a CCTV image of the man they need to speak to about the incident..
Detective Constable Wayne Green said: "The man stripped off his clothes and ran up and down the aisle completely naked.
"The man clearly thought this was fun and a joke.
"In reality, this was completely unacceptable and inappropriate behaviour which was clearly distressing for many of the passengers on board what was a busy service.
"If you recognise the man pictured, or know his whereabouts, then we'd like to hear from you.
"If you are the man pictured I'd urge you to come forward and take responsibility for your actions."
Anyone with information should contact British Transport Police on 0800 40 50 40 or call Crimestoppers, anonymously on 0800 555 111.