Edith Nesbit 
The Railway Children [EPUB ebook] 

A young family must leave their comfortable London life for a house in the country when their father is taken away unexpectedly. Roberta, Peter, and Phyllis spend their days at the nearby train station, befriending the station master and porter and many of the passengers. Waving goodbye to the departing London train each day and sending their love to their father who is detained there, they strike up a friendship with a kindly old man who may be able to help them get their father back.