The 35-year-old jewellery dealer had put the case on the luggage rack and realised it was missing when the train arrived at Rugby.
A suitcase containing rubies, emeralds, and sapphires worth £1m has been stolen from a London to Birmingham train.
A jewellery dealer boarded the 7.03pm Euston train on Wednesday 8 November, travelling to Birmingham New Street.
He had a suitcase with him which contained more than 40 precious gems.
The 35-year-old had put the case on the luggage rack and realised it was missing when the train arrived at Rugby.
Detectives believe the bag is most likely to have been stolen from the train before it left Euston.
Police want to speak to a man who was seen on CCTV in connection with the theft of the large black suitcase.
“I’d also like to hear from anyone who was on board the train or at Euston station on Wednesday evening, who may have seen a man acting suspiciously,” investigating officer Detective Sergeant Nick Thompson said.
:: Anyone who has information or recognises the man shown in the CCTV image is urged to contact British Transport Police on 0800 40 50 40 or text 61016, quoting reference number 529 8/11/17.