OK, time for some "Swiss Politeness 101".
Train, going for a vacant seat:
You make eye contact, then *silence*
Swiss opposite gives you a non-verbal, aggressive, "what!?"
That's because you've broken the ritual.
Ritual: "Isch dä Platz no frei?" "Cette place est prise?" FOLLOWED by eye contact.
a) Swissie mumbles something indicating it's free barely looking up.
You take the seat. You *both* ignore each other. (If we're in the mood to chat, we'll chat, if we're not, we won't, something to do with private thoughts, busy reading a book/newspaper or just in the mood to vacantly stare out the window. It's nothing personal.)
b) Swissie speaks in full coherent phrases, makes eye contact and starts making social noises.
Jump on it! This one's not busy and in the mood for some chitchat.
Out for a walk in the city:
You greet, nobody greets back.
Um, we're not out here for fun or for social opportunities.
We're busy! Going from A to B, shopping, catching the train, thinking about which route will get us fastest, in the limited time available, through whatever it is that we have planned.
Greeting is then an intrusion, an obstacle to navigation.
Out for a walk in the woods:
You greet, (guesstimate) 2/3rds greet back.
-> We're *not* busy, we're on leisure time, enjoying life etc.
-> We're busy (hiking! last bus to catch!) but doing something we enjoy.
The 1/3rd not greeting back: plain ignore them.
Fact is, for a foreigner it can be a bit hard to recognize if whoever you encounter will greet back. I usually go by body language and apparent laugh wrinkles around the eyes.