Having noticed the title of this thread, but also the (now) length of same, I will only comment in an (old school personal general sense), not having read all the comments above...
When I joined the job, and had to 'commute' to get to work (1970s), there was an unwritten rule (amongst regular commuters) that everyone respected everyone else (in the carriage concerned), and this largely revolved around people sitting quietly reading their papers, or catching up on lost zzz time, but in essence most carriages were quiet on the way in to work (even if all seats were occupied), there may have been odd conversions going on between a few people, but this was at a very hushed/low key level. That was how it was in my experience. And if in any doubt when reading this, this was on the SWML of the Southern, Basingstoke inwards towards Waterloo. It was a basic instinct amongst most, to have respect for others around us.
As a newbie to the job at the time, but perhaps more so, a newbie to the etiquette of 'commuting' this was something that has stayed with me ever since. A basic RESPECT for those around me, something that in more recent times seems to have been lost by many people, both in railway commuting terms and more generally in society.
I wonder if this latter day (perceived lack of respect) is a generational thing, or is a result of techy (ie social media) mediums?
Having had conversations with some from a younger generation than I, in a wider (than railway) sense, I do find that many/most are very respectful in their outlook in general, which leads me (personally) to think that the problem in general lies with the modern social media world, something I have for one have suspected ever since the invention of the internet... but I'm sure others may have their views?