Agatha Christie's Great Detectives Poirot and Marple S01 E21 4:50 from Paddington, Part 1: The Train a Murderer Rides

Elspeth McGillicuddy, a friend of Miss Marple, witnesses a murder on a passing train while on her way to visit St Mary Mead. The police stop investigating when they cannot find the body. Mabel persuades Miss Marple to send her on a dangerous undercover mission to find out the truth.