I understand that there's more than one way to do a braking system. The idea that a single failure point could disable the friction brakes, though, seems a bit odd given the general "fail-safe" philosophy otherwise used in railroads and rail transit. There's no rule requiring air brakes (which themselves can fail if used improperly) on rail transit vehicles, but I suspect there will be some poking around to find out why this particular BART problem occurred and how widespread it is. Probably a failure mode that nobody though of before.

There does seem to be some regional difference in transit rail braking preferences. For instance, it seems like practically all the rail transit Back East uses air brakes, including DC Metro and NY subway and some PCC cars. Other systems (all-electric, electric + hydraulic in BART's case) are more common (though certainly not universal) Out West. Many of the air brake systems are blended with dynamics, so it's not a technical issue.