Specifically I’m looking at the Katowice to Vienna (Wien) international route. This is one of the routes that is subject to “Oferta Super Promo International” - there are PDFs available that show this pricing applies to most, if not all international rail routes into Poland. It seems this was started in early 2022.
Normally on the PKP/Intercity booking site https://www.intercity.pl/en/ , when booking a domestic train you can select a Eurail “reservation only” from the Discount/reservation type dropdown list on the selected train. No problems, it works as far as I’m aware.
If I select almost any train a few days ahead from Katowice to Wien Hbf on the PKP/Intercity website, there is no option to select Eurail, I just get the message that it’s a special offer with a tick-box that cannot be deselected. I need to reserve two months ahead and cannot see any way around this.
Polrail say they can reserve a seat, but their pricing to buy a full ticket on this route bears no relation to the PKP published fares, it makes me think that the Polrail system may be out of date. If I look at prices a week ahead, Polrail is 387PLN for a full ticket, or 48PLN for a reservation. On the PKP website the price for a full ticket is 47 or 90PLN depending on the train selected, no reservations available. There are no refunds on the promotional tickets.
Full lists of the PKP promotional fares can be found here Super Promo International - www.intercity.pl