On a train or flight I will acknowledge them and you can tell if they might have a chat or not. .
There is a strange law though, where people who you really don’t want to talk to once they get start, are the keenest to start and rarely take a hint of I’ll just read my book for a bit or rest my eyes.
Best and most interesting conversations have generally been on T bar ski tows. I can’t abide queuing and will always jump on the free space or invite any singleton behind to join me. Apart from reducing any queue, it’s surprising what a small and interesting world it is at times when you chat to people.