The Pennsylvania Trolley Museum (located about a 35 - 45 minute drive south of Pittsburgh near Washington, PA) has in its collection a stainless steel Budd-built rapid transit car which operated on the Market-Frankford Subway-Elevated line in Philadelphia. Nicknamed an "Almond Joy" car due to the bumps on the roof, car #606 was donated to the museum in 1998. Although the museum has track to operate its collection of streetcars and interurbans, this car is on static display.
Here's a link with further information:
Market-Frankford Line Car 606
Even though the car does not operate, I am glad it has been preserved, because it represents an era of rail transit history (1960s) not well represented in most transit museums' collections.
If transit enthusiasts and historians believe that the BART cars are historically significant, hopefully one or two will be preserved, even as static displays.