Why is this the answer?

The answer is railroad because this amazing riddle is playing a fun trick with words by comparing the parts of a train track to the bones in your body. When you hear the words "backbones" and "ribs," you probably picture an animal or a person, but the riddle is actually talking about a strong structure made of metal and wood. It is a puzzle that describes an ordinary object in a brand new, imaginative way. Let's be detectives and look at the first big clue: "Only two backbones." In your body, your backbone is one long, strong column of bones that helps keep you standing straight. A railroad track has two long, straight pieces of metal that run side by side for a very long distance. These two shiny metal pieces are called rails. Just like a backbone supports your body, these two rails are the main support that the whole train rides on. They run parallel to each other and are the strongest part of the entire track. That is why the riddle calls them "two backbones." Now for the second clue: "thousands of ribs." If you look closely at a train track, you will see many pieces of wood or concrete laid sideways underneath the two metal rails. These flat pieces are called railroad ties or sleepers. Their job is to hold the two rails at the perfect distance apart and help keep the entire track stable, so the train doesn't wobble or fall off. Because there are so many of these ties lined up one after the other, they look just like the many "ribs" in your body that wrap all the way around. Since a railroad track can stretch for hundreds of miles, there are truly "thousands" of these ties holding those two metal backbones firmly in place. It is a perfect way to describe a strong and useful path for trains.