People waiting on platforms are the perfect subjects for drawing, and since humans are a serious weakness of mine, drawing randomers is great:
I have a major thing for drawing trees, that's been apparent in most of my sketchbooks since college. Even more so when they're often the most common sight on my train journeys home. Whether or not the sketches below look like trees is debatable but I'm rather fond of using a bamboo dip pen to create quick scratchy forests.
Dip-pen & Ink.
One of my studies from our uni trip to the Victoria & Albert Museum, London - There were a lot of human figures in that sculpture!