Mr. Alden, aka Grandfather, arranges a big surprise for the Alden kids in the final chapter: He's moved their beloved boxcar into one of his gardens. Everything's the same as it was in the woods, except now the boxcar is next to a fancy fountain instead of a brook. Also, the kids can just play there instead of, you know, living in it. The children are super excited to see their old squat, and Mr. Alden is excited that they're excited, so basically, everyone wins.
Oh, and of course, they all live happily after. The book even goes so far as to come right out and say as much:
"Thank you for the surprise, Grandfather," said Violet. "We'll never go away from you again."
"I hope not, my dear," said Mr. Alden. "We'll all live happily ever after."
And so they did. (13.66-13.68)
It's worth noting, however, that the end of this book is not the end of the Boxcar Children's adventures. In fact, there are more than 100 mysteries in the series, though the original author only wrote the first 19.