You don’t have to say anything. “Good morning.” “Goodbye.” Both are optional. Why say farewell to someone you’ve barely met?
Maybe because “barely” means “scantily,” but scant is not zero. Some addition has happened, but will the sum be more than its parts? That’s up to you.
What if “barely” means “only just,” and you don’t yet know where that train is going? Well, you’re already on it! You may as well get to know your fellow passengers.
The best reason to say “Ciao,” however, is that your courageous step into uncertainty might make a stranger’s day. A pioneer rarely sees her work come to full fruition, but she trusts that it will – and we’re all the better for it.