When using public transport there are so many people with earphones in listening to music, or looking at their phones, entertaining themselves during their commute. I often use this time to practise drawing my fellow passengers or other things that catch my interest. In fact I rarely use gadgets when commuting and either sketch my surroundings or if without some tools, think about how I'd sketch something that catches my attention.
To do so I use a small notebook that is easy to fit in a pocket and thus more likely to be used. (I find it has to be practical or I wont do it!). Additonally the small size is less conspicuous allowing me to draw in relative obscurity and blend in with the crowd.
While I may use pencil I often use a trusty old ball point pen for my sketches. I find this trains the brain to dismiss the notion that the next mark may not be 'right' but to just assertively go for it. One must also be pragmatic, this is only a sketch, I'll be doing another soon, if it goes wrong its not a biggie.
There are of course problems - the fact that you are in motion, lots of unplanned jumps and bumps mean you cant be too precious about any one drawing. Also a passenger may get up and walk off at any moment. So I draw quick, deciding rapidly what I want to concentrate on, a particular part or the whole person.
Why do I do this you may ask. For one, practise. The more you draw the better you get, finding time to do things is a pressure we all struggle with and I now see journeys as extra sketch time. In fact I do it in waiting rooms or anywhere I may have the opportunity. Secondly, us artists like to exercise creativity and with other pressures in life its good way to get a creative fix. Thirdly, As any artist will attest, when painting or drawing time passes quickly, the same can be said for sketching during my travels, long journeys pass much faster!