The Railway Children, by Edith Nesbit
When Father goes away unexpectedly, Roberta, Peter, Phyllis, and their mother have to leave their happy life in London for 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. But the mystery remains: where Father, and will he ever return?
I just recently read this book and really liked it! I'd never read any E.Nesbit books before and I'll have to read more. It is definitely a nostalgic book, capturing kids of that time & culture. I don't think any modern, American children would every docilely accept the mother's request to not ask any questions why their father had been taken away (as well as not do their own investigation). But I think back then, kids accepted that adults had their own world that did not effect theirs...