The railway children - E. Nesbit

When Father is taken away unexpectedly, Roberta, Peter, Phyllis and their mother have to leave their comfortable life in London to go and live in a small cottage in the country. The children seek solace in the nearby railway station, and make friends with Perks the Porter and the Station Master himself. Each day Roberta, Peter and Phyllis run down the field to the railway track and wave at the passing London train, sending their love to father. Little do they know that the kindly old gentleman passenger who waves back holds the key to their father's disappearance.