Musician Stephen Morris, 51, forgot to pick up the £250,000 instrument when he got off at Penge East station in south London last week.
Detectives have now discovered another man took the violin and left the train four stops later at Bromley South.
The suspected thief then boarded a connecting train to St Mary Cray.
“Officers believe the man in the CCTV image may have information which could help their investigation,” the force said in a statement.
The antique violin is believed to be only one of a few made by David Tecchler, whose name is marked inside, in Rome in 1709.
It was packed inside a white glossy case along with two valuable bows when it was left on the 11.58pm service from London Victoria to Orpington on 22 October.
Mr Morris, who has been a violinist for 44 years, said he was devastated to lose the violin, which featured in Harry Potter and James Bond film scores, and appealed for its safe return.
“I had done a very long day at Abbey Road recording and I was just exhausted and I’ve left it on the train,” the musician, who lives in Sydenham, southeast London, told The Independent.
“It is devastating – nobody’s died and it could be a lot, lot worse – but on a personal level it’s pretty massive.”
He had been due to play the 310-year-old instrument while leading the Royal Philharmonic Orchestra at the weekend.
Anyone who recognises the man in the CCTV image or has seen the violin can either contact the BTP on 0800 40 50 40 or Crimestoppers on 0800 555 111