The book doesn't end with one of the kids' visits to their grandmother but instead with a more grown-up Joey (or Joe, as he's called in his adulthood) passing through Grandma Dowdel's town on a train.
He's off to do his training and then fight in World War II:
Grandma was there, watching through the watches of the night for the train to pass through. She couldn't know what car I was in, but her hand was up, and she was waving—waving big at all the cars, hoping I'd see.
And I waved back. I waved long after the window filled with darkness and long distance. (8.6-8.7)
This moment—where both Joey and Grandma Dowdel are waving to each other—is poignant because it shows that there's still a thread of connection there. Aww, shucks.
Even though Grandma Dowdel doesn't know if Joey can see her, and even though she can't see Joey, she still waits up all night to wave at him. And he waves back because he loves his grandmother and wants to say goodbye to her before he goes off to war.