That is the number for Caltrain Transit Police.
To everyone who reads this in the City, this number should be in your phone's address book: +1 (415) 553-8090.
That is the number for SFPD Dispatch.
To everyone who reads this in the City, or the East Bay, this number should be in your phone's address book: +1 (877) 679-7000.
That is the number for BART Police.
If you see anyone go on to the track—who (for Caltrain) is not crossing and going on their way—you need to call the above number—or 911—immediately. Be sure to provide a location relative to a station (such as "100 feet North of Redwood City station"), or the city name and nearby street. Transit Police in particular, I believe, have a direct link to Caltrain Dispatch, who can make certain signals (normally those near switches/crossovers) fall to red, or at least warn nearby trains.
Similarly, when you're standing on a platform, you should not be looking down at your phone. Listening to music, fine, but you should always be looking around. If for no other reason, than so that you can make sure you don't accidentally get bumped by someone else.