The name of the manor house where Marple conducts her inquiries was called Rutherford Hall in Agatha Christie's novel. This was changed to Ackenthorpe in the film to avoid comparison with the surname of Margaret Rutherford (Miss Marple).
During the opening journey on the 4.50, Miss Marple reads "Death Has Windows" by Michael Southcott: this was a fictitious novel mocked up by the production team. Towards the close of the film she has a Fontana paperback copy of "Murder is Easy" by her creator, Agatha Christie.
Miss Marple brings her golf clubs to Ackenthorpe Hall and uses the ruse of hitting golf balls hither and yon to allow her to explore the grounds and outbuildings, looking for the body of the woman she saw murdered on the train. She tells young Alexander that she won the Ladies Open Handicap in 1921.