The train can symbolize many different things, depending on what you are taking away from the story.  The story is essentially about a kid who wants respect from others, and goes about it in all of the wrong ways.  He thinks that owning a gun and having power over others as a result will earn their respect; instead, he would have received respect if he had taken responsibility for his actions and worked hard to pay off his debt.  For Davey, the train symbolizes escape, and a chance at a new life; he feels that will for sure earn him respect.

The train could symbolize rebellious youths who shirk responsibilities.  It could symbolize society's unwillingness to work hard.  It could symbolize excuses or rationalizations for bad behavior.  It could symbolize society's degrading morals. It could symbolize the power of technology over family.  It could symbolize industrialism's takeover of the younger generation.  The possibilities are many. I hope that those ideas help to get you started; good luck!