A somewhat lengthy journey on a train (or down a river like Huck Finn) often isn't just about physical movement; there's usually some sort of figurative journey as well. And in this case, the trip from Bright Sands Beach to Orleans is about moving from the past to the future. But you don't have to take our word for it. Check out this passage:
He looked back the way he had come. The twin rails of the train tracks were being swallowed by the dense jungle. Every minute on this train took him farther from his past.
He had to smile. His whole body hurt, but he was alive and his father was in the distance and whatever lay ahead, it had to be better than what lay behind. (15.23-24)
Nailer's life is, quite frankly, not too awesome. His dad's a total jerk who is willing to kill his own son, plus the work Nailer does ship breaking is dangerous, and there's no guarantee he'll be able to do it much longer. It's not that hard for Nailer to leave this all behind and tie his fortune to Lucky Girl. So as he travels to Orleans, he's physically leaving but also mentally and emotionally embracing a major life transition.