"4:50 From Paddington" has one of the most memorable sequences in Agatha Christie history: while on a train, Elspeth McGillicuddy (Pam Ferris) sees a murder in a passing train. The investigation then leads her friend Miss Marple to Rutherford Hall, a house with an extremely disfunctional family.
"4:50 From Paddington", though having many liberations from the original novel, is definitely the best of the bunch, its competator being "The Murder at the Vicarage". The episode has been criticized for bad acting, but I think it's great. Even though they don't have people like Jo Lumley and Derek Jacobi, I think it's very good. Some people think Rutherford Hall was filmed at one house, but this is not so. It was filmed at at least four different locations, really making Rutherford Hall their own creation. The ending is also very well done, as well. My advice: see it.