I think that Boyne includes this description for a couple of reasons. The substantive reason becomes that Boyne wishes to draw a stark contrast between how the Nazis live and how their prisoners live. Bruno makes it a point to note the fundamental difference in what he sees. One train is comfortable, seats available, and represents "a different life." The other one is more crowded, armed guards present, and there is a fundamental sense that more difficulty is intrinsic to the other train. Both trains are going in the same direction, yet Bruno notices the difference. Such a description helps to bring out how Bruno's perception of "the other" is evident from an early point in the narrative. This will continue, as Bruno will become more aware and more conscious of the differences in his world and this "other" world. Through his own acknowledgement of this difference as well as his commitment to minimizing this difference, this will become a force of resistance evident in his character. The differences between both worlds will become collapsed and merged into one at the end of the novel by Bruno's own choosing, helping to illuminate the transcendent theme of how differences between people, if in the wrong hands, can become dangerous. Boyne's description of this early on helps to establish a part of Bruno's characterization that will be a major part of the novel as it develops.