BART is there, it's the way it is, and any new cars have to match the others for wheels etc. They're aluminum because the others are, too, because it's lighter than stainless (reduces power), and because it lasts about as long as you want it to (just like stainless). Certainly, it doesn't rust.

Anyway ... with 3 doors, it looks like BART is finally getting over its ambitions of being a comfy suburban railway and getting with the city subway vibe. Yes, they do have a lot of long trips, but they also have a lot of short one, probably more in terms of boardings than the long ones. Transit systems live on boardings. When 3-door trains eventually become dominant, they might even reduce the length of some station stops, which could speed up service a tad. Welcome to Big City Transit.