I feel that on charter/heritage trains, bars across the windows could be enough. Locking the windows 'up' could also work, as Mk.1s and Mk.2s used on those trains have opening regular windows in the saloons/compartments. Adding interior door handles probably isn't necessary as there can be staff at the stations, after all, they're not exactly commuter trains. Regular passenger trains with regular passengers, it's a safety hazard. I wish it wasn't, but unfortunately a few idiots ruin things for the rest of us! I've great memories of sitting on my bag in the vestibule of GWR HSTs in the summer with the window down running through beautiful countryside. Not sticking my head of out it at 125mph though!