Train Etiquette

If you're sitting on a train in a window seat and the person in the aisle seat next to you is reading his newspaper with his nearside elbow periodically prodding you in the ribs, what are you supposed to do? Particularly if you shuffle a bit to nudge his elbow to make it clear he is invading your space and he totally ignores it? Obviously the dark neutral stare doesn't strike fear into quite everybody I encounter.