Officers investigating the theft of a valuable violin on a Thameslink service travelling between Brighton and Bedford have released CCTV images in connection to the incident.
The theft happened at approximately 5.20pm on Tuesday, June 11, according to British Transport Police.
The victim was heading towards Haywards Heath when they accidently left their bag on board containing the violin, police said.
Another man then approached and took the bag, leaving the train at East Croydon station.
Officers believe the man shown in this CCTV image may have information which could help them investigate, a spokesman said.
They said: "If you know who he is, contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 498 of 11/06/2019.
"Or you can call Crimestoppers anonymously on 0800 555 111."