I've been commuting on the train for a while now and have been using two programs on my iPad for drawing and sketching. Each has their advantages and disadvantages. SketchbookPro is extremely full-featured and lets you save in Photoshop format with layers for later tweaking. Paper provides the most fluid drawing experience I have ever experienced. Paper also provide a standard "brush" that makes every stroke you draw just feel artistic.

Anyway, here's a recent sketch of a young woman on the Greenline -- this one is called Orange Girl on the GreenLine. I really like her sneakers. 

You can see more of my sketches here

Comments are welcome and ideas for other drawing programs and devices are always appreciated.