These digitally enhanced drawings were initially done many years ago during my commuter rail days. The trick was to do them without staring at people, otherwise they'd find another seat or cover their face with a newspaper. I liked the roughness and spontaneity of having to work fast, and using a black marker forced me to keep the accident as part of the work. I find that a combination of spontaneity and accident brings life to a work.
My book on Train People is available on Blurb.com