I have found that public transport offers plenty of different photo opportunities, from entire train stations to people and small details. Everything from wide angle lenses to telephoto lenses can be useful. Even if I just happen to carry my camera from some event, no matter which lens that happens to be on the camera, it is likely that there is something to photograph.
(The timing of this post is not entirely arbitrary - until recently I had the one month regular public transport ticket - starting with the Snowball week - but now I’m again primarily bike-bound, so fewer public transport photo opportunities for me.)
The big picture / Wide angle
Using the subway, passing escalators is normally a part of the journey. Several photo opportunities. Many of mine turn out dull, but now and then the lines and/or the colours create a pleasant visual effect.

Colourful escalators
Since I don’t care about the documentary aspects as I’m photographing for fun, I have no objections to manipulating the colours the way I want them - which might mean anything from over-saturation, like the one above, to black and white.

As a winter bonus you get the moist-lens softening for free if you come in from the cold into a warm subway hall (the camera manuals advice against this usage, especially if you do not have weather sealed lenses).

Converging Lines
Movement
Large vehicles rapidly moving - opportunities to practice your long shutter speed skills:

... either from the outside ... (Photo is from Häggvik tramway station, shutter speed 1/30 sec.)

...or from the inside! The photo above is from inside a German bus going from Dagstuhl to St Wendel. The driver drove at least 10 minutes with the doors wide open before realizing something was odd (we didn't complain, as it was rather warm, and no children in danger on the bus - in fact, me and my cs summer school travel companion D were the only passengers). The shutter speed 1/50 sec was enough to create the blur.
People
If you have the guts and/or don’t mind risk being seen as obtrusive, public transport is also good for candid portraits. (Since I
do mind being seen as obtrusive, there are many moments where I later regret not having the reflexes to snap a picture.)

Waiting for the subway

Security guards
The two pictures above are taken with my 135mm lens, which allows you to get close without physically being close.


In the line for the bus
On the other hand, travelling nighttime on weekends you might also encounter people that
want to pose for you when you carry your camera visibly...

Random posing people
Details
Finally, there are plenty of details which can be worth paying attention to.

STOP

Refresh Yourself
All of the photos in this post are taken from my compiled
set of public transport pictures