Metal Train Homepage (German only)
This is basically a private train that goes from Stuttgart to Itzehoe on Wednesday and back on Sunday, with stops in a few cities (Koblenz, Mainz, Mannheim, Köln, Essen etc, I don't remember all of them). The cost of MT shuttle bus trips to and from Wacken is already included in the train ticket. And the train itself has a luggage wagon, a bar wagon and a disco wagon, basically "full service"... though it is not too cheap (was to expect, ne?)
Here you get the ticket, which is then exchanged for a special MT armband (make sure to keep it throughout the festival, you will need it to get back!!).
What else I can say (as MT passenger since 2004) - its organisers DO learn with the time (as opposed to you-know-who); this year it was almost perfect.