"The Mystery of the Blue Train" by Agatha Christie is a detective novel published in 1928. When an American heiress is found strangled aboard the luxurious Le Train Bleu bound for the French Riviera, her priceless ruby missing, Hercule Poirot must unravel a web of unhappy marriages, secret lovers, and jewel thieves. With multiple suspects—including the victim's estranged husband and a notorious count—and conflicting testimonies, Poirot discovers that nothing aboard the Blue Train is quite what it seems.
Wikipedia page about this book: en.wikipedia.org/wiki/The_Mystery_of_the_Blue_Train