Well, this thread has prompted me to have a good old think, and I'm grateful to the OP for that, discussion and debate are the only practical way yet devised for us humans to shuffle along together without resorting to violence.
As has been pointed out a number of times above, objective measurement for the purpose of enforcing any legal rule is effectively impossible. Lawyers might make some money from it, but it would be unlikely to make many people happier.
Some of the posts here have, in my view, fallen prey to the widespread assumption that things are getting worse. I have to confess I'm prone to a bit of that myself. But I cast my mind back to the early 1970s, when I was a quite frequent user of the rail system. As has already been observed, the very concept of a quiet coach in an era of jointed track, opening windows etc would have been somewhat optimistic.
But were fellow passengers really that much more accommodating of other people's wishes?
First, there was the universally assumed right to smoke. A habit never taken up by more than half of the population, with pernicious effects on bystanders far beyond irritation at somebody producing sound, was nevertheless inflicted, fag butts, ash, smoke, smell and all, on everybody, even in the often-pathetically-insulated non smoking sections. I remember as a kid being trapped in a suburban full-width compartment (ie no possibility of escape whatsoever) in winter rain (no option to open a window) with seven or more enthusiastic smokers. It was horrible. Mind you, that and the thick treacly yellow patch on the ceiling above my granny's armchair managed to counteract the "glamorous" images with which we were bombarded, so hey ho.
Then there were Inter City journeys. It really did seem in those days as if buffet cars actually came equipped with half a dozen half ****ed squaddies stood in the corridor, complete with cans of Younger's Tartan, an abundance of fags, and a range of salacious chat up lines for your older sister, rapidly turning to hostile abuse if she didn't respond "nicely".
Then there were journeys on a Saturday in the football season! I think I've probably erased some of them from my memory. There was actual blood at times, and back in the early 70s, a general feeling that there was nothing much you could do about it, but avoid if possible.
All of which makes me think that a discussion of the concept of a public-library-on-wheels might need a bit of perspective.