My experience has been that establishing eye contact generally allows one of the people involved to begin speaking. In the scenario you mentioned of two people on a train, if one of them is absorbed in a newspaper and never looks up, no talk takes place. That is barring some kind of calamity, such a the train suddenly going around a curve throwing one of them to the floor or worse onto the other one, then talking ensues for sure even if it is only profuse apologies. I think that is why eye contact is considered SO important. Without it, no communication of any sort happens. Even if its only to wave at someone, eye contact must be made first. Since so many of us have trouble making proper eye contact, its understandable why we don't have many conversations. Likewise, that is why wearing shades is such a relief for us. Even if we look at the other person, they don't know it so no attempt at greeting or conversation results.
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Your neurodiverse (Aspie) score: 120 of 200
Your neurotypical (non-autistic) score: 74 of 200
You are very likely neurodiverse (Aspie)
AQ = 38 MBTI = ISTJ Gender = Non-binary
I strive not to perseverate. You can PM me for more info.