I didn't see any signs of a taxi around the station when I was there. I was there about the same time, end of May.
Werfen is a very small town, about 3000 people. A couple got off the train with me and said they were walking up to some ice caves.
When you walk from the train station to the castle, you'll come to an expressway (Salzachtalstr. - Rte 159). You can walk along the shoulder up to the castle, or you can cross Salzachtalstr. and take Marktstr. through town. Salzachtalstr. is a little shorter, but Marktstr. is more interesting (and safer).
Hohenwerfen castle, by the way, is the castle that was in the background behind the grassy slope in Sound of Music when Marie first taught the children to sing.