Westbahn have historically treated Interrail passes as second class tickets. Even if you had a first class pass, you had to pay to ‘upgrade’ if you wanted to sit in first class.
Based on their website, it seems this has changed, and they now accept them in first since 15/9/26:
https://westbahn.at/en/news/interrail-eurail-pass/
Reservations are optional, but still incredibly expensive - the same price as the previous upgrade.
The Rail Planner is still showing that first class passholders need to pay a supplement - I think this needs to be changed
