how does shoveling the snow from the railroad tracks bring the war closer to the boys?

The war finally intrudes with the boys' first experience with hard physical labor at the railroad depot in Boston. The once-white snow is now "drab and sooted, wet and heavy," quite a dramatic contrast with the pure white snow that had covered Devon; the image conveys how reality has become more pressing and more bleak for the boys. They gradually drop their "fresh volunteer look" as the day's work goes on; the change from boisterous young men to strained-looking laborers ushers in the influence of war, and foreshadows the change that will come upon them when they finally go to war.