Letter from an unknown passenger

In this brilliant scene, everything but the male passenger is deceptive. Years later the man will receive a letter from an unknown woman. The letter will begin in the following way:
'To you, who have never known me.
My boy died yesterday.'
The man from the train will then read about the life of a woman, in which he had unknowingly played the decisive part. More, he was the very heart of that life.
When he will have read her last words, shadows of what was once a memory will rise in him. The letter will fall from his hands.
The scene is from Letter from an unknown woman (1948), a film directed by Max Ophüls and based on a short story of the same name (1922) by Stefan Zweig. The short story (with the complete letter) can be found here.