OBB Railjet between Salzburg and Innsbruck

Last summer I took a OBB Railjet between Salzburg and Zurich, Switzerland. I believe it was on the part of the trip between Salzburg and Innsbruck where we appeared to go underground and then picked up a lot of speed. Is this a stretch of track that is in an underground trench that is dedicated to high speed trains? If not is there a long tunnel on that section of the route?

Yes, between Wörgl and Hall im Tirol there are several very long new tunnels, but they are not specifically for the Railjets. Rather they are to get the heavy traffic using the Brenner Pass line out of Jenbach and the small towns along the old line.