4:50 from Paddington

S1 E3: Two trains draw briefly alongside each other as they race past a country house. To her horror, Miss Marple's friend Mrs McGillicuddy witnesses a man in a passing carriage brutally strangling a mystery woman. With no body to be found, the police decide she must have dreamed the incident. Miss Marple is convinced the answer lies with the ill-fated family who own the mansion and persuades a bright young friend to pose as a housekeeper and gather clues for her.