In the novel 4.50 from Paddington, Mrs Brierly is mentioned by Miss Marple. Mrs Brierly had a very bad nervous breakdown, and was advised to go to the mental hospital for observation. However, she was worried about her children knowing about it, so she wrote fourteen postcards, and arranged for them to be sent from different places abroad. She then told her children that she was going abroad on a holiday.
Inspector Craddock had mentioned that the murdered woman could not be Anna Stravinska, because one of the girls from the ballet company had received a postcard from her saying she was on a cruise, and had arrived in Jamaica. Miss Marple mentioned Mrs Brierly to make the point that anyone can get a postcard sent from anywhere, and so the postcard did not really prove that the murdered woman was not Anna.