As above: shoooting up heoroin or the like. Called so because a session will leave a dark linear mark (known as a "track") at the site of the affected vein. Harcore users will tend to have multiple sites of injection and will locate, or "spot" an optimum vein - one with minimal "tracks" and discomfort or infection. A hit can be analogous to the impactof a locomotive or train.
That's how I figure it anyway.
Watch the damn movie. It rocks. try reading the book - the narration is spelled with a scotish accent all the way through.