London Underground is a wonderful mixture of opposites in this respect.
During the very crowded morning rush, it is almost a capital offense to make a sound (except perhaps some noise escaping from headphones). You can forget about talking to another passenger, even a friend. Even looking happy while not obviously reading something that might be funny is met with embarrassed glances at feet at ceilings.
In the evening though, after about 9 or 10, the tube is full of groups on their ways home from the pubs - then there are people singing, often whole carriages will silently giggle at someone being an extrovert centre of attention.
Finally, there is the etiquette surrounding when one must offer a seat to someone else. the order of priority appears to be:
* pregnant and (obviously) disabled people, plus new mothers.
* old people (ladies first)
* children (especially medium sized ones who look like they might actually sit
down, and/or shut up if given a seat)
* the rest of us, in a reserved, passive aggressive scrum.
(also, obviously, talking to a complete stranger on the tube is a massive faux pas, no matter the time of day...)
On quiet coaches specifically, I have to say that on British "mainline" trains there is a big problem, which is reservations. For example, on virgin trains there is always a quiet coach (both a standard and first class one), and a coach with no reserved seats. If you want a seat at a busy time of day, you must be one of the first to arrive at (on Virgin it's coach E) and take one (it is even possible to get the prized window-side-facing-forwards-at-a-table seats this way.) However, these coaches never coincide. Thus, if you want to be in the quiet coach you must book a seat in advance, which is cheaper but restricts you to a particular train - if you're late you must pay twice, and if you're early you have to wait. This is important when you're on the tube (which frequently has delays) and the trains from Euston to Manchester are every 20 minutes (and absolutely packed in the evenings, especially Fridays, in spite of this.) Given the convenience factor, I usually plump for coach E, and risk the screaming babies/mobile chatterers - as another commenter pointed out, this is why I paid £80 for a pair of headphones.