The Railway Children — Edith Nesbit / E. Nesbit

This classic -- adapted for radio, television, stage, and screen -- “has a special and enduring place in British children's literature” (The Guardian).
 
Roberta, Peter, and Phyllis used to live in London. But their father, who worked at the Foreign Office, was accused of spying -- and now he's in prison. So they've moved with their mother to the Yorkshire countryside, to a house near the railway called the Three Chimneys.
 
They may have much less money than they did before, but the siblings still have a great deal of adventures, and soon they make plenty of new friends: the “Old Gentleman,” who takes the 9:15 train, plus the station porter, the stationmaster, an engineer, a fireman, and a mysterious Russian man who steps off the tracks with a remarkable story to tell.
 
Before long, the Old Gentleman will do something very special to help his young chums -- and the children will have their own chance to be heroes as well . . .