I will share one bit of warning to you about a really bad experience we had on one leg of an OBB train trip in Austria. On our travels, one stop was in Melk along the Danube. After visting Melk, we were scheduled to take a short train ride early one morning from Melk to St Polten where we would then catch the train from St Polten to Salzburg. The St Polten to Salzburg leg (2 hour ride) was booked as 1st class. The train from Melk to St Polten was late, which caused us to miss the train from St Polten to Salzburg. While we were waiting on the late train to arrive in Melk to take us to St Polten, we called an OBB customer service agent. This was all on the phone. He booked us on a later train from St Polten to Sazburg, but he told us there weren't any seats available in 1st class and that he had to book us 2nd class seats. He sent us the new 2nd class seat assignments via email.
After we arrived in Salzburg and checked into our hotel, we called OBB customer service about getting a refund for the cost between 1st and 2nd class seats given we were not able to get in 1st class. Austria is just like the USA, it's beauracratic and requires filling out a form (smiles). We submitted the required form on line while in Salzburg and now we've gone back and forth with them for several weeks. They've finally rejected our request for that refund (this just happened this morning). They said their records show there were 1st class seats available and that we were supposed to get a "non-use form" from the OBB ticket person on the train to prove we did not use the 1st class tickets. So what we learned is don't trust what you are told on the phone when rebooking. Double check when you get on the train. And most importantly if you are bumped out of 1st class because of a rescheduled train, make sure you get a "non-use" document. The OBB ticket inspector on the train went through the 2nd class cars and actually checked our tickets. He even mentioned we were originally booked in 1st class. We explained the late train leg and how the phone agent said there were not any 1st class seats available and that he had to place us in 2nd class seats. The ticket inspector just nodded and moved on to the next person. He didnt mention or offer us what is called a "non-use" document and we had no clue it existed or was needed or we would have asked for it. It's left a bad taste in our mouths to the point we aren't sure is next time we would likely just rent a car. Still, our overall trip to Austria was fantastic. The vast majority of our trains were on time, and easy to manage. Just be cautious if you get bumped out of 1st class.