Fun (to me, anyway) new feature just now:
In any popup when you click on a connection, if you click on the "X Travelers" line and subsequently hover over it, you get the list of travelers of that segment. If you hover before clicking, you get instructions to do so. It's done this way to avoid what seems to be an expensive operation to get all that info up front, but really quick to get for individual segments on-demand. Open to ideas of a better UI, subject to the requirement that the user would need to take some action to trigger loading of the data to prevent lots of apache and DB hits to load info that wouldn't be used in many cases.