4.50 from Paddington

Two trains.
Side by side for a brief moment.
In that moment, a murder.
Elspeth McGillicuddy is positive she witnessed a man strangling a woman to death.
But it was only the merest glimpse through a carriage window as the trains drew parallel.
She is the only witness, there are no suspects, and, most importantly, there is no corpse.
Who, apart from her friend Jane Marple, would take her seriously?
Never underestimate Miss Marple.