I bought a one-way 2nd class ticket on oebb.at website Salzburg HBF to Wien Westbahnhof. It's an early morning departure at 6:00 and I arrive into Vienna at 8:24. When buying the ticket, I don't remember seeing the option to pay extra for a reserved seat. On the Railjet ticket, I don't see details for seating assignment #, or car #. Where do I sit without occupying someone else's "reserved" seat? Do I just sit in any one of seats in the 2nd class? Thanks in advance Marlene
I don't recall if ÖBB uses a digital dispaly or paper, but either way, if someone reserved the seat, they mark it. Likewise, per your other question, on the DB ICE trains, you'll see a small digital display above your seat that notes for which portion of the trip you reserved the seat. You may find someone sitting in the seat already, but in this case, you are within your rights to request them to leave.
That time of day, it should not be a problem. Both Oebb and Westbahn have trains between Salzburg and Vienna about every hour, and most riders will pick a later train. The one time we took an early train out of Salzburg (to catch a connection in Innsbruck), there weren't more than a handful of people in the whole station. Besides, if any train fills up, it is more likely to be Westbahn.
If you really worry about it, you can buy seat reservations at the train station prior to travel.