"There are a few direct trains (one in the morning and three in the afternoon/evening) that take about 2:15 in total. In the middle of the day, there are two options that include changes and take about 3:15. The Swiss Rail site has all the details, but you'll need to use the Austrian Rail site to see the timetable for the return trip."
Finally, someone with real information rather than the usual spouting off with no firm facts. Oh, right, that was me.
It looks like Swiss/Austrian rail have opened up more direct trains between Zurich and St. Anton for the winter. I just checked for January 5, and there are direct trains running all day long starting at 10:40. (That's means no more triple-change journeys for me if my flight is late and I miss the 10:40 train.)