In the beginning of the story Mrs.Stevenson was trying to reach her husband at work because he was working late and she was nervous about being alone all day.
Then her husband was planning with another person to murder his own wife because he didn't like her anymore and had a very detailed plan.
Mrs.Stevenson was very shocked at what she heard and wanted to stop it from happening and wanted to tell the police and the phone operator to see if she can help.
Mrs.Stevenson was very nervous and was trying to call the police to tell them what happened and try to stop it from happening but the cops would not believe her the operator wouldn't believe her either.
But then at 11:15 the train went by and the woman got scared because she new that someone was going to get murder because the train passed by so they couldn't hear her scream so they wouldn't call the cops.
All in all, Mrs.Stevenson was the person who got murdered even though she thought that someone else was going to get murdered but it was actually her.