I'd expect that not much is won by adding more and more weight to the vehicles in a system like BART where all vehicles are essentially the same weight. They could and should of course be engineered to as to protect riders from impact and structural failure.

In systems where heavy trains are mixed with light ones, I can see a benefit in beefing up the lighter vehicles. However, don't heavier vehicles generally mean higher operational costs, besides the increased purchase price? That cost could instead be applied towards Positive Train Control, which has been shown to virtually ELIMINATE catastrophic collisions between trains? The German accident happened because the freight train ignored a red light in a location that is not yet equipped with such a system, which is rare.