As Easy As Falling Off the Face of the Earth by Lynne Rae Perkins

Picture this. You decide to go to camp in Montana (you live in Wisconsin). On the train you open up a piece of mail from said camp to learn the camp is not going to be open when you get there. Your train breaks down. In an attempt to get a cell phone signal to call home, you get off the train, despite being told not to. While you are trying to call home, the train is fixed and starts to move. Without you.

Now what?

Well, if you're Ry, you walk to the nearest town (hours away). You meet a friendly man Del who offers to drive you back to Wisconsin. Meanwhile, your grandfather (who's staying at your house in Wisconsin) takes the dogs for a walk and falls into a pit in the woods. And your parents, on their first vacation in forever in the Caribbean, leave their cell phone on a volcano.

What other option does Ry have but to find his parents and his grandfather?

A good book, although some suspension of disbelief is necessary. Ry is a great character, flawed but not too much. He's the main story, although his grandfather, his parents, and even the dogs get a chapter or two here and there. A good road trip and adventure story.