Since starting my train commute, I have made this trade: two hours by train in place of 45 minutes by car. Net result: I am more relaxed when I get to work, I have already read The Book of Laughter and Forgetting by Milan Kundera (of The Unbearable Lightness of Being fame, but I'm told this book is the same only better), and I have fixed two DBMail bugs during my commute. Some days I get caught up on my email by leaving my laptop on overnight so that it picks up my new messages, reading and responding to them while on the train, and sending the messages with the wireless at work. On yet other days, I edit IETF drafts for the RFC Editor, which means that I've earned a half a week's train rides because I get paid it!
Of course it's only been four days, so basically I've just done each of the above once or twice, but I can just tell already that these will be the things I do every day.
Best of all, I don't have to be alert enough to deal with other drivers!