Edith Nesbit 
Railway Children 

When their father is mysteriously imprisoned, Roberta, Peter, Phyllis, and their mother have to leave their happy life in London to live in a small cottage in the country. There they take comfort in the nearby railway station, and discover that the gentleman who takes the same train every day might be the key to proving their father’s innocence…An enormously popular story about the joy and excitement of being young, E. Nesbit’s captivating classic will enchant readers of all ages.