I think people answering these comments are from other countries that don’t understand that on a train from Reading to London in rush hour, there might be 60 seats and 80 passengers per carriage. 20 of these pax standing despite their ticket that said “Feel free to sit on any free seat you happen to find!”
Great western railway operates like this, offering you a “mystery seat” regardless of whether they’ve sold 120% of the seats in the train.