I've spent a lot of time on the train in and out of London the last year and a book makes it far less tedious. There is something really soothing about reading on a train, plus it doesnt make me feel sick like in the car. I did once get on the wrong train because I was engrossed in a book though!
If I am reading a really heavy book then I usually take something a bit less hefty with me instead, even if I don't finish it.