4.50 train from Paddington - Agatha Christie

The two trains passed side by side, and one of the trains slowed slightly before accelerating again. And at that fleeting moment when the windows of the two trains lined up, Elspeth witnessed the crime. The man (whose back she saw) was squeezing the woman's neck with all his might, and the woman soon fell dead, her face congested. Then the train picked up speed and went out of sight.

But who would believe this story but Miss Marple? Especially when there were no evidence, no witnesses, and no body!