The TTC is pretty mellow as transit systems go (comparing to Los Angeles, Montreal and London, UK). So I find I’m able to block out distractions and focus. I also stand in the doors that don’t open until my stop with my back to the car and my headphones in, so the distractions are minimal. I would not do this if riding after 11 pm alone, but it’s fine during the day. I do find that the sense of motion I get with the train helps my momentum. It also works as a kind of timer and as a forced writing exercise. I have nothing else to do, so I might as well do that.
When in LA earlier this year, I did notice that the subway is a) a lot noisier b) very active and c) that there are a lot of characters. Toronto commuters have a don’t look/talk/eat/stare policy that probably helps a lot.