How to hide a sketch in plain sight

As I commute to Wall Street I draw my fellow passengers. But it is rather awkward when they catch me staring at them. What can I do?

1. wear a hat with a big brim. No problem, I own a Tilly hat. But the Tilly hat inevitably starts a conversation with some hearty type asking me what river rafting or mounting hiking or glacier gliding I've done (almost none, I like the hat)
2. wear sunglasses (but I only have the clip on ones, not cool)
3. get over the embarrassment and draw anyway.

There is an invasion of personal space that an artist must negotiate to make images of other people. At the Art Students League, models are paid to pose. It is understood I will stare at them and make marks on paper or canvas. But the commuters probably have the right to tell me to bugger off...they didn't agree to model. But I want to find a way to do these quick studies. I'm inspired by other commuting sketchers on the subway. Drawing from photos is rather, uh, flat. Reality yields more as the subject shifts, thinks, nods off, moves their hand while holding a book, cellphone, rosary beads, a child's backpack...they hide and reveal parts of themselves and morph through perspectives...as an artist I can capture the movement and select the most telling expression. But did I capture their expression or project mine? Maybe it is an indefinable both. And here is my stop. Gotcha.