There is very little in the text of A Separate Peace that is not imagery of one sort or another. Knowles carefully crafted his words to build and reinforce the themes contained in his story at every opportunity.
When some of the boys from Devon volunteer to shovel out the railroad yard, the effort is approached in their minds at first as a working holiday - an opportunity to make some money while helping the "war effort" that was immortalized as being "a bore." Upon arriving at the railroad yard, however, they discovered not pristine white snow but "drab and sooted, wet and heavy" snow that quickly became exhausting to shovel. When the first train was finally able to travel through the newly cleared tracks, the boys prepared to celebrate their victory over the snow with the passengers on the train.
It was a troop train....They were not much older than we were and although probably just recruits, they gave the impression of being an elite as they were carried past our drab ranks. They seemed to be having a wonderful time, their uniforms looked new and good; they were clean and energetic; they were going places.
The contrast between the students, grimy and sweaty in their safe school existence, and the soldiers, looking rested and crisp in clean clothes as they headed off to war, creates an image that reflects the contrast between the peace of Devon and the war that is highlighted throughout the book.