From what I recall, the rails themselves are magnetic. I don't think they use any other fixed magnets. But the coils I keep talking about charge the rails and make them uber-magnetic. The armature just slides back in forth in the barrel and is only repulsive to the magnetic field generated by the rails.
I'm pretty sure that's right. The presentation was two years ago. I may have forgotten a lot since then, but I'm faily sure that's right.
I'm just looking up a website for everyone to get a little more info.
Head over to the wiki site: http://en.wikipedia.org/wiki/Electromagnetic_gun
The "Theory" section is very good.