Okay, but for example Stadler's Metro can be configured such that it's lighter than the original BART stock. (I don't believe there's currently any in operation

Okay, but for example Stadler's Metro can be configured such that it's lighter than the original BART stock. (I don't believe there's currently any in operation in the US; most of what's in the US is FLIRT/KISS based, which are decidedly heavier stock designed for non-rapid-transit usage.)