It happens almost once a week that I find myself on a train mere feet away from someone I know, usually from church, and who I will not interact with at all during our 10-20 minutes of riding public transit together. We won’t look at each other, and we certainly won’t speak. I mean, I will look at them, but only long enough to make the “incidental” eye contact that would make it acceptable to smile in recognition and say hello. Then I look away. I don’t know, while I’m looking away are they trying to make incidental eye contact with me?
This feels strange. I fear I’m being rude by not greeting people I know and that they’re going to recount it to someone later, like, “I was on a train with him for eight stops and he didn’t even say hi.” But I follow this self-imposed protocol for fear of being intrusive. We all seem very occupied on public, mostly with our phones but also with books or magazines. We have perfected the inaudible signal that says, “Don’t speak to me.” To break into someone’s personal time during their commute feels uninvited and irritating.
We are a city teeming with people, strangers to one another in the most crowded of spaces, even among friends.
YoRocko! 
Oh, good grief, say hello. If the person doesn’t want to talk, they will nod, or say hi, and leave it at that. No wonder we are becoming such a lonesome civilization if this is how we treat people we KNOW. How do we treat those we don’t know?
This is very thought-provoking. I usually have my “nose in a book,” in my mother’s often-used phrase, to the extent that the buses’ audio cues call me out of the story. But some of my more memorable rides have resulted when someone notices the book, says it looks good, and wants to ask about it. If I like it well enough to read it on the bus, chances are I like it well enough to recommend it. Now if I could just figure out what signal to find in others to find those recommendations!