Maybe it's different in other schools, but seems to me the word tranquility isn't often spoken in Zen centers. The Buddha's teaching on tranquility do permeate Zen; it's just the word that doesn't get said much. Maybe tranquility sounds boring, or passive, or something.
The connotation of passivity may be what turns some of us off to the word "tranquil." There's a silly caricature of the wise man living serenely, and remotely, on a mountaintop, detached from the rest of the world. The serenely detached act may impress the tourists, but what does it teach? and who does it help? Dharma is better actualized in the messy, here-and-now world, and most practitioners come to understand that pretty quickly, I suspect.
The Buddha included tranquility (passaddhi in Pali; prasrabhi in Sanskrit) in the Factors of Enlightenment, and it's also associated with the dhyanas, so it's an important component of the dharma path. It is compared to the stillness of a pond on a windless day.
The last post looked at stress, a modern concept that is primarily physiological. Tranquility in Buddhism is a mental factor that relates to all the skandhas except the first one. That's an important point, I think, because it brings home the understanding that "mind" is not just something that lives in your brain.
The Buddha's advice on cultivating tranquility includes eating a healthful diet, associating with congenial people, and living in a pleasant climate. I'd say the climate issue didn't slow the Tibetans down much. But put into modern terms, he seemed to be saying that one's entire lifestyle should be conducive to tranquility.
This ties into Right Lifelihood; being a floor trader at the New York Stock Exchange is probably not helpful, tranquility-wise. It's probably a good idea for all of us to review where and how we spend our time and consider what might be keeping us agitated. Reducing stress may not be the same thing as being tranquil, but it's not a bad thing to do. And you don't have to move to a mountaintop, I don't think.