Edith Nesbit 
The Railway Children 

When Father mysteriously goes away, the children and their mother leave their happy life in London to go and live in a small cottage in the country. »The Three Chimneys» lies beside a railway track — a constant source of enjoyment to all three. They make friends with the Station Master and Perks the Porter, as well as the jovial »Old Gentleman» who waves to them everyday from the train. But the mystery remains: where is Father, and will he ever return?