Why Do The BART Police Have Guns?

On Jan 1, an unarmed man was shot and killed while lying on his stomach on a BART train platform in Oakland. I don’t think I have anything substantial to add to what has already been said, except to ask this: why do the BART police carry guns?

It may surprise Americans to learn that even bona-fide police officers do not routinely carry guns in many countries, e.g. Britain. While I can certainly imagine public-transit situations that would require deadly force, other countries seem to have sorted out unarmed policing, even on the city streets.