The system has been clean for the past two decades, and I'm given to understand that it never really went through the "vandalised and thuggy" phase that the NYC subway had. And you're quite right ... MetroRail tolerates no crap whatsoever of any kind. NYC Subway adopted a similar policy some years ago, and is supposed to be much improved over it's grafitti covered, litter filled hellhole days in the 1970s and '80s.
People still eat and drink to their hearts' content on the NYC subways.