A Lesson in Love (1954)

After 15 years of marriage, David and Marianne have grown apart. David has had an affair with a patient of his and Marianne has got herself involved with her former lover Carl-Adam, who's also David's best friend. When she travels to Copenhagen to meet Carl-Adam, David takes the same train as she does, making it look coincidental. Spending time together remembering their past and talking about their future, they come to understand each other again, which leads to a reconciliation.