I envy you, TorNorth.  That must have been a tease of an experience, but still sounds like a good one.  I would have not been as polite as you, however, and at least turned around for a while just to catch the slightest glimpse.

I enjoy these kind of observations - when you had a quick look did she have any visible tears on her face?

I remember seeing a woman crying on the train a few years back and was amazed (and pleased) to witness tears rolling down her face unchecked.
My wife has cried in public a few times and she has also allowed tears to fall unchecked - I love it when women tell their tears flow.

I asked her why she didn't wipe her tears away - she didn't really know why she just said I was upset.

These kinds of moments are both exciting and frustrating.  I have one myself, from 10 years ago, where I was too polite to stare at the crier (if I knew back then how much crying would mean to me later in life and how rare observations would be for me, I would definitely have made more of an effort to steal a glance!  Not to mention a hug as well!!)  But the one upside is that, even if she wasn't a lip curler, you'll never know that, so you can just imagine her however you like.