The Railway Children, by E. Nesbit

An ebook, free from Amazon for my Kindle app. An old book, now in public domain. A sweet, old-fashioned children's book. It features three upper middle class British children, Roberta ("Bobbie"), Peter, and Phyllis ("Phil"), who live comfortably in a London suburb with their mother, father, a cook, and one or two maids. Life is good until one evening when Father is suddenly called away. He does not return, and some time later the children and their mother move to a small house in the country, with no servants. They no longer go to school, and their mother no longer has the time to play with them, nor tell them stories or make up sweet and silly poems for them. At first she tells them they must play at being poor, and as time goes on, she tells them they are poor. They all miss their father, but no one mentions him and the children are in the dark as to why he disappeared. They entertain themselves by waving at the trains as they pass by near their home, and visiting the kind porter at the railway station. One adventure leads to another; they save lives; they prevent a train wreck; they rescue a boy who has broken his leg in the tunnel. They are polite and respectful and kind to everyone they meet, and everyone is charmed by them. As one might expect from a children's book, it ends happily; Father is reunited with his family, but they have grown stronger, braver, and wiser in his absence.
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This is a better, more thorough and more thoughtful review of the same book by my friend at Reading to Know:
The Railway Children