Routine.

• Any sleep you're getting on a train isn't typically REM sleep, as such, you're constantly "waking up" for a couple of seconds only to lose consciousness again.
• You're more likely to experience these "awakenings" when at a station, when noise and movement are at their peak.
• During "awakenings", you check your surroundings and compare it to where you should be.
• Over time, you will automatically create "warnings" that give you advance notice of your stop - a particularly busy station, for example. Once you hit your "warning", you typically keep yourself awake for the last leg of the trip.
• For mass transit, you typically follow a schedule. Your body will become used to naturally timing when it should wake up from these "naps." For example, my "nap" was between 35-45 minutes, as my trip was 50 minutes long.

Source: Slept on DC Metro for 45 minutes each morning/evening, 5 days a week for 2 years, missed my stop three times in total (never by more than one stop). All three times were because I was off-schedule/no sleep the night prior.

As a side-note: you're relying completely on other people for your own safety and security, and make yourself a target for theft (particularly if listening to music on a smart-phone). It's not something I'm proud of.