In Japan you'll find that on the ride home many people engage in escapism via different forms. MP3 players are just a supplement in some cases, the real interest can be texting on the phone, a nintendo DS or PSP game system, a good manga (comic book), a laptop.. often the MP3 player is not the only thing there. If it is, I can bet you the person is probably sleeping.
When I was living in Japan I was given an iRiver as a b-day present. At first I wasn't really into it because I preferred to study, but as I got used to it I did use it more to enhance my train riding experience. I found certain songs were uplifting for me in the morning, in general my mood dictated what I listened to. Usually upbeat stuff in the mornings, trance or jungle, or some classic rock, and then some more relaxed stuff on the way home. Like reggae, dub; anything down tempo. I also had music on when I was on my way to the gym. That was much more ritualistic, I had a certain song I listened to before I trained. My train ride was about 20mins each way to work. I'm half Japanese if that counts