There isn't much different about bart.

Basically everything on BART is unique to BART. Odd loading gauge. Odd track gauge. Lightweight aluminum chassis so none of the aerial infrastructure is designed to carry a heavier, more traditional car. Multi-part wheels with aluminum hubs to make extra noise. Non-standard traction voltage so BART struggled mightily to find replacement thyristors for the old cars. BART still struggles to keep their electric substations running. The original signaling system makes some sense, but trying to replace it with another NIH product in the 90s made zero sense. The original trains had glass with compound curves and BART could not find a vendor who could reliably recreate them. The current trains indicate ADA seating with the color green, reserving blue for the regular seats.

If it can be made differently or done differently BART will absolutely try to do so.

  the real problem is thinking they are different or that they need to innovate. 

Yep NIH is a huge problem for BART.

I've heard that makes procurement way more difficult, you can't just order a train car in the standard gauge from many manufacturers. It's like big and tall sizing, yes, any place can make it, but there just isn't that large volume to create a liquid market.