A slang term for the Captain (O-3) rank in the Army and Marines. The term comes from the Captain rank insignia, which is two wide bars connected by two thin cross-bars, resembling railroad tracks crossed by two railroad ties.
Damn, Lieutenant Carter used to be really cool, but ever since he pinned on his railroad tracks, he's just pulling rank and treating all his old buddies like shit.
You can't go to the office in those railroad tracks, Bill! Thanks for offereing to iron your own pants, but if they turn out like that you'd better leave it to me.