The Mystery of the Blue Train — Agatha Christie

Bound for the Riviera, detective Hercule Poirot has boarded Le Train Bleu, an elegant, leisurely means of travel, free of intrigue. Then he meets Ruth Kettering. The lovely American heiress--bailing out of a doomed marriage--is en route to reconcile with her former lover. But by morning, the young bride's private affairs are made public when she's found murdered in her luxury compartment. The rumor of a strange man loitering in the victim's shadow is all Poirot has to go on. Until Mrs. Kettering's secret life begins to unfold...