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?