So as train travel becomes more prevalent and more time sensitive users (commuters who couldn't possibly catch a later train) start using busier trains, where will everyone's breaking point be where you get off at the next stop or invite those people who are space invaders to do one?
Mine will be when people are sat in the out of use seats either between window seats or next to me because there are no other seats or they might be standing / congregating in a vestibule or aisle.
Exactly as I said some posts back - the train companies are now actually creating confrontation situations and potential violence ('invite those people to do one') between passengers. They may be present anyway, but actions such as marking seats out of use aren't going to help things.