In chapter 10 of A Separate Peace, Gene makes a very difficult and uncomfortable visit to Leper, who has been discharged from the army because of mental instability. Leper lived out in the country in Vermont, and Gene had to take a train ride and then a bus to get to the town he lived near. After that, he had to walk the rest of the way, because "there was no taxi," and "there was no one...who would offer to drive [him] out there." This journey across the New England countryside, on train, bus and foot, reminded the adult Gene of all of the traveling that he did in the war. In war, soldiers are often shipped across unfamiliar terrain in foreign countries; his journey across Vermont reminded him of all of those trips that he took as a soldier. He didn't know Vermont, and he was going into a frightening situation (seeing Leper), just as he went into frightening situations in the war.
The book as a whole is a metaphor for war, and Gene compares his visit to Leper as one frightening battle in that war. When he visits Leper, he realizes for the first time how dramatically and horribly war can impact people's lives; for one of the first times, war becomes very real and personal to him. So, his train ride out there, to the realization of war's impact, was much like the train rides he took in the actual war to actual battlefields. I hope that those thoughts helped; good luck!