4.50 from Paddington movie

Elspeth McGillicuddy, an old friend of Miss Marples, witnesses a strangulation murder in a passing train through the window of her coach compartment. When Police Inspector Slack cant find a body along the tracks and doubts the validity of the story, Miss Marple and Elspeth locate the area where the body was probably thrown from the train, the isolated manor house of an aged, cantankerous millionaire, Luther Crackenthorpe. Miss Marples niece Lucy is recruited to apply for a job on the estate in hopes of locating the corpse. When she finds it in an ancient sarcophagus in an outbuilding housing Egyptian and Etruscan art treasures, the police and Miss Marple are brought in. Obviously someone in the large household is involved, but who?