That's what this guy did, and wow do I hate him for it. He wrote a whole 400-page novel on his smartphone while going to and from work.
"I got very fast at writing with my thumbs. I found myself writing more and more." Soon, he was averaging 400 words each morning and evening. "I trained myself that at 8 a.m. and 5 p.m. every day when I got on the train, that was my writing time," he said.
So his book (a demon-laden fantasy, the first of a series) hit the bestseller lists and he quit his day job and now writes full-time... but he still takes the F train. "He insisted his word per minute count is higher on the subway than at his desk at home, where distractions include the Internet and a 9-month-old daughter."
When I was working at Smart I used to eat up 2+ hours a day just commuting back and forth, from QC to Makati. Oh, the unwritten books! Of course I'm a freelancer now, with theoretically (I emphasize the theoretically) more free time on my hands... But, you know, those comics aren't going to read themselves. And there are TV series to catch up on. Priorities, man.