The Spanish version of the Spirit Tracks website is unique, because unlike the English-language versions, this one actually provides the real name of the villain of Spirit Tracks, the Demon King. By reading the rest of this post, you may be subjected to spoilers.
Yes, indeed – not only does it call him by his title “El malvado rey de los demonios” (the evil king of the demons), but it also calls him by what seems to be his real name: Mallard. If you search Google for Mallard you’ll find that not only is it a train, but a famous one – it is the fastest steam train ever made.
This enhances the possibility that the Demon King and main villain of Spirit Tracks could be a train – more specifically, the evil-looking train from the trailer seen escaping after the destruction of the Spirit Tower. It makes sense, after all – if the shackles holding him down are train tracks, then he is probably a train-themed villain.
Some were hoping that Ganon was the Demon King, but his name is certainly not Mallard, so at this point there seems to be no mention of him. We will have to wait and see, however, since Ganon has revealed himself as the villain behind the villain several times before.