This is one of Agatha Christie’s finest works, paying homage to the locked room genre while adding some surprising new twists. There’s really no locked room here, merely a train carriage on the Orient Express halted by snowfall. But the constraints are similar: A murder takes place, but during a time when no one could have entered or left the train. The suspects are a motley assortment of passengers, but each new clue seems to make it more unlikely that any of them could have committed the crime. I’m not surprised that this story has been adapted for film, TV and radio on so many occasions. Even by Christie’s elevated standards, it’s a masterful work.