Masterpiece Theatre continues its 30th anniversary celebration with a perfectly charming, beautifully photographed and excellent acted production of “The Railway Children,” a two-hour drama based on E. Nesbitt’s children’s story. The book was first published nearly 100 years ago and — as a testament to its popularity — it has never since been out of print. The story centers on a London family — mother, father, two daughters and a son. When the father mysteriously disappears one evening, mother attributes his absence to “business.” But father doesn’t return, and mother and the children are forced to give up their house, sell their furniture and move to a rundown cottage in the country. Mother begins to eke out a living for the family by writing stories. The cottage holds few charms for the children, but what grownups might consider a drawback the children regard as an asset: A railroad line is not far away. They take to furiously waving as trains pass — convinced that their “hello” message will somehow get to their absent father. Soon an elderly gentleman in the train’s rear car begins to wave back. When mother falls ill with influenza, the children decide to ask the elderly gentleman for help. They meet the train as it stops at the village station and hand him a note specifying their needs: beef broth and brandy for their mother. Soon, a trunk filled with food and supplies arrives at the ramshackle cottage. The gentleman turns out to be a director of the railroad. In short order the children repay his generosity by preventing a derailment with signal flags they hastily make from the girls’ red petticoats. The children’s adventures along the railway line continue, but they never swerve from their mission of saving their father from whatever his dilemma might be. Eventually, of course — this is a children’s story — they succeed. Poverty is far from ugly in “The Railway Children,” but that’s not the point. Family devotion, pluck and self-reliance are the central themes. The story is helped by a strong cast: Jenny Agutter (“A Respectable Trade”) as the mother; Richard Attenborough (“Jurassic Park”) in his TV debut as the elderly gentleman; and Jemima Rooper, Clare Thomas and Jack Blumenau as the children. Rooper is one to keep an eye on. She has a face the camera loves and is remarkably self-possessed for her age. “The Railway Children” is a delight for the whole family and richly deserves its place in the lineup as Masterpiece Theatre celebrates 30 years on the