Not much of a plot to "The Plymouth Express," but it still has a top cast and fabulous production values.
An old friend, Gordon Halliday, asks Poirot to check out a man interested in his daughter Florence, but Poirot does not take that kind of case and refuses. However, when Carrington reminds Poirot that he is not a parent and can't understand the pain of seeing his daughter with someone unscrupulous, Poirot relents.
Florence boards the Plymouth Express with her jewel box and is murdered. A devastated Halliday asks Poirot to help solve it. There are a few suspects, one being Florence's ex-husband, Rupert, who asked her for money. Another is Armand de la Rochefort, the man Halliday didn't like. Actually, he doesn't like either man.
Poirot knows they won't find the murderer until they find the jewels.
A neat mystery, very clever, and if it was a little padded, so what? It's entertaining, with Poirot, Hastings, and Miss Lemon, and the denouement is good. These mysteries have class -- better to pad a Poirot episode than something less well done.