I mentioned this in another post but it's worth posting separately.
Austria is one of those countries where there are more than one train company. There are two different rail companies on that Salzburg-Vienna route and their tickets are not interchangeable - you are either Team OBB or Team Westbahn. Per the Man in Seat 61 they are essentially the same and Rick agrees, his only thought was that Westbahn can be cheaper if you're buying day-of tickets. That was certainly my experience a few weeks ago. Oh and you end up in a different train station depending on your train company. If you're on a pass, you must use OBB. There were announcements on my Westbahn train reminding customers that OBB and DB tickets were not acceptable on the route.
You can buy tickets from a machine or from a human, there can be a slight surcharge for the latter. Trains have different classes, the Man explains this in depth. When I bought my Westbahn tickets the seller automatically gave me second class, I started to ask about the differences and she said once you're on the train look around, if you like first better you can pay the conductor. I thought second class was just fine. There were vending machines onboard (which don't take Amex BTW), you don't really want to sit too close to them since they tend to attract the youngers who are having a party. There are displays at each end of the train car telling where the train is going and what the next station is, and often your speed which is so cool.
When your train pulls into a station, don't dally - be up and at the door with your luggage to exit, or standing on the platform near a door waiting to board. They don't stop for very long.