Hard seating is IMHO the worst thing about modern trains (well, modern transport in general). A lot of it comes from cars where hard seats are also common, but then they can be adjusted for height, back pitch, back support, headrest position, and sometimes more. Train seats, particularly standard class, are just fixed, so anyone who doesn't fit the standard dimensions isn't going to be that comfortable. They also need you to sit in a specific way for comfort, which is a problem as people like to relax on trains, even go to sleep. Again, in a car, the occupier of one of the seats, which in many cases is the only occupant of the car, is not in any place to relax or sleep, infact it's better for them to be less comfortable and less relaxed. I do wonder if a lot of it has to do with how people today are coming with more of their own padding (fat) so don't need so much padding on the seats?