This book is another English classic.  The story concerns the Waterbury family who move to "Three Chimneys", a house near the railway, after the father who works at the Foreign office, is imprisoned as a result of being falsely accused of selling state secrets to the Russians. The three children, Roberta (Bobbie), Peter and Phyllis, find amusement in watching the trains on the nearby railway line and waving to the passengers. They become friendly with Albert Perks, the station porter, and with the Old Gentleman who regularly takes the 9:15 down train. He is eventually able to help prove their father's innocence, and the family is reunited. The family take care of the Russian exile, Mr Szczepansky, who came to England looking for his family (later located) and Jim, the grandson of the Old Gentleman, who suffers a broken leg in a tunnel.

According to Wikipedia, the theme of an innocent man being falsely imprisoned for espionage and finally vindicated might have been influenced by the Dreyfus Affair, which was a prominent worldwide news item a few years before the book was written. And the Russian exile, persecuted by the Tsars for writing "a beautiful book about poor people and how to help them" and subsequently helped by the children, was most likely an amalgam of the real-life dissidents Sergius Stepniak and Peter Kropotkin who were both friends of the author.