The Secret Vanguard (1958)

The Secret Vanguard is a 1940 thriller novel by the British writer Michael Innes.[1] It is the fifth in his series featuring John Appleby, a young Detective Inspector in the Metropolitan Police. The novel takes place in the early stages of the Second World War, and functions closer to a mystery spy novel than the traditional detective novel.
Synopsis
Appleby's investigation of the murder of a poet seems to hinge on something the dead man overheard on a train journey shortly before. Meanwhile while crossing the Forth Bridge on another train, Sheila Grant hears a strange exchange between two men with a snatch of misquoted poetry. Later on she is kidnapped and held a prisoner in the countryside close to Troy Castle in the Scottish Highlands, apparently because it is now feared she knows too much. By this stage Appleby has worked out the two cases are connected and launches a major manhunt for her.