This was one of a series of stories contributed to the Evening Standard by different authors under the main title “Murder on the 7.16.”
An apparent suicide has taken place on a train from Bologna to Florence, but Commissioner Spadoni is suspicious. Signor Marani, a jeweller, appears to have shot himself while the train was in a tunnel with no lights in the carriage. Spadoni interviews the three other occupants of the carriage, Signora Monzio who sold her jewels to Marani, Captain Zampi who had bought jewels from him, and Signor Betti who was Marani's business partner. Has one of them committed murder?
Commissioner Spadoni, by the way, is the name of the police investigator in Canning's 1952 novel Venetian Bird, set in Venice. I think this is just Canning being economical with names rather than trying to set up a recurring character.
Evening Standard, Thursday 29 September 1955, p. 19.