Sentimental jewellery was among £1,000 worth of items stolen from a bag on a train near Peterborough Station.
The theft took place on a service between Leeds and London. At approximately 3.45pm on Saturday, October 20, a man took a piece of luggage from the racks and went into the toilet.
In the toilet the man then removed a number of items, including sentimental jewellery. The items have a collective value of approximately £1,000.
The man left the bag in the toilet and it was found by another passenger after the train had departed Peterborough.
Officers have now released a CCTV image of a man they believe has information which could help them investigate.
If you know who he is the contact British Transport Police by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 1800077486.
Or ring Crimestoppers anonymously on 0800 555 111.