Gertrude Chandler Warner 
The Boxcar Children [EPUB ebook] 

Four orphaned siblings, determined to stay together as a family, decide to inhabit an abandoned boxcar and turn it into their new home in the woods. When one of the siblings falls ill, the others go out to seek a doctor to save their sister. Meanwhile, their wealthy grandfather has offered a sizable reward to anyone who has information about his grandchildren, hoping that they can all be reunited once more. As the children continue to evade being found out, will they ever know peace?