Everything and nothing. It depends on one's interest or alertness to perceptions and surroundings. One specific thing I learn every time I ride a bus or streetcar is the minutia of people's lives, their emergencies, their stresses, a lot of things I'd rather not learn; but they talk loudly into their cell phones about personal things. They swear and threaten people on their cell phones. Subways are potentially quieter.
We have a bunch of new electric buses on rails. Not a lot of cities still have them the rails. Anybody else still have streetcars on rails?
This is a nice image. So I learned that sitting on a bus when it starts is
Like being inside the stomach of
A growling beast
Interesting locations and transit systems in that link. The downtown Toronto streetcars are not exactly what you'd call a "light rail system" but they would have a lot in common with some of them. The new streetcars pictured above do have that light rail look. Unlike downtown Calgary, where for the most part, the commuter trains are very much separated from car traffic, the tracks from the Toronto streetcars are not separated at all. (Somewhat like the San Francisco Cable Cars but not as hilly.) Consequently if you are in a hurry on a major n-s or e-w street, and you get behind a streetcar, it takes some maneuvering to get out in front. Streetcar drivers often get off the bus to yell at car drivers who pull up when their doors are still open. It's messy and gritty and there's a lot of mixing it up, unlike the pristine bus ride that flagamuffin was on above - which I suspect was 100% metaphorical.