Agatha Christie herself considers this her worst book. The Mystery of the Blue Train was written in 1926, probably one of the worst years of her life: her unsuccessful marriage followed by her mysterious disappearance, and her mother's death. The content and the unravelling of the crime itself also reflect some of the complications concerning the human condition and relationships.
In the back street of Paris, a famous ruby, Heart of Fire, is changing hands. This piece of jewellery has a long history of bloodshed and tragedy trailing after it, and is said to bring bad luck to its owners. An American millionaire, Rufus van Aldin, buys the ruby and gives it to his only daughter, Ruth Kettering, who is unhappy in her …
Lee, Amy. "The Mystery of the Blue Train". The Literary Encyclopedia. First published 11 July 2003
[https://www.litencyc.com/php/sworks.php?rec=true&UID=117, accessed 21 January 2018.]