There is that 1 compartment with 3 seats. I imagine you could buy 3 tickets and reserve the seats to protect your privacy. I’d imagine you can book the 3rd or 4th ticket for “a friend” that did not show up . If the train is packed beyond belief I guess someone might try and take your empty seat and you would then need to start explaining… but I’d think this not terribly likely.

Another option would be to book the First Class seats and once you get on the train check out Business Class. If it still has empty compartments you could move there and pay the fee onboard. Of course someone else could still show up in the middle of the trip.

if you are taking a RailJet into Czech Republic then it might be a Czech RailJet which has open 1 - 1 seating.