I watched a film or play on BBC TV when I was a child. I am now 46, so it would be over 30 years ago. The plot involved a group of people in a train carriage. We the audience know that one of them is a murderer, we just don't know which one. As the journey progresses, they gradually leave the carriage leaving only an old lady and a young man. The young man is then murdered by the old lady.
I have tried tracing the program to no avail. Please help.

Many thanks.