Depends on which station you will be stopping at. I have been to the Orlando station many of times and you can usually call up a cab service company right on the spot (We used Mears). The fare is sually $37 to $50 depending on station, etc.
The last time we hopped onto a shuttle van. I think the company was FL Tours and that was $35 for 2 people from Orlando Amtrak to Animal Kingdom Lodge. The only bad part is they have to make stops for the other passengers along the way.
Right now there are no "official" WDW transportation from Amtrak to WDW.