From the BBC:
A BBC full-cast radio drama starring Maurice Denham as the great Belgian detective. A millionaire strikes a deal on the seedier side of Paris and gives to his heartsick daughter, Ruth Kettering, the "Heart of Fire," one of the world's legendary jewels. Legend has it that the possession of the jewel leads to death - but despite her father's protests, young Ruth decides to take it with her on the Blue Train to the French Riviera, where she has planned a reunion with her former lover. A few days into the journey, Ruth is found dead and her priceless rubies stolen. The suspects line up: could the murderer be Derek, her weak and greedy husband; Ruth's former lover, the Comte de la Roche; or the beautiful but tempestuous dancer Mirelle? Only master sleuth Hercule Poirot can unravel the mystery and find the truth.