He also mentions that the train is "full of people" and that they are all "going somewhere" but he doesn't know where. He is fascinated by the train and spends a lot of time watching it from his window.
Bruno's description of the train is significant because it reflects his childlike innocence and lack of understanding of the world around him. He doesn't understand the gravity of the situation, nor does he understand the purpose of the train or where it is taking all the people.
This innocent perspective contrasts with the harsh realities of the Holocaust, which the train represents. The train is a symbol of deportation and death, but Bruno sees it as a source of excitement and wonder. This juxtaposition between Bruno's perception and the reality of the situation adds to the emotional impact of the novel.