4:50 From Paddington By Agatha Christie

For an instant the two trains ran together, going in the same direction side by side. In that frozen moment, Elspeth, riding in the one train, witnessed a murder in the other. Helplessly, she stared out her carriage window as a man remorselessly tightened his grip around a woman’s throat. The body crumpled. Then the other train drew away.

Who, apart from Miss Marple, would take her story seriously? After all, there were no suspects, no other witnesses... and no corpse. Not the police.