<p> For an instant the two trains ran together, side by side. In that frozen moment, Elspeth witnessed a murder. Helplessly, she stared out of her carriage window as a man remorselessly tightened his grip around a woman?s throat. The body crumpled. Then the other train drew away.</p> <p> But who, apart from Miss Marple, would take her story seriously? After all, there were no suspects, no other witnesses . . . and no corpse.</p>