Use an alarm on your phone in vibrate mode. After a while you won't need it.

The other trick is to keep your naps under 20 minutes. After 20 minutes you fall into a deep sleep, making it harder to wake up and leaving you feeling groggy after you do wake up.

Guess it depends, when I used to travel to the office every few months I was lucky enough that the train would terminate at my destination. So I could sleep in safety.

I've certainly observed commuters set alarms on their watches/mobiles based on their expected arrival times. I find that a little rude, but nobody else every seems to comment..