Having a conversation, and then realising that you're providing amusement for everyone else on the carriage, as otherwise everyone just sits there in complete silence.
Finding exactly the right place to stand on the platform so that you don't have to walk at all when you get to your destination (and pondering where the optimal place to stand is when going to an unfamiliar destination station).
Waiting two minutes for the next train, and getting a seat instead of piling into a crowded carriage.
The sheer inanity of the announcements: "we're being held at a red signal", really? what colour do you use for "go"?