I know what it runs on! It's a 5' 6" in gauge, usually used in India, and used no where else in the US.
The larger engineering lesson from that is you're probably better off making standard solutions work for your situation than custom solutions. The wider gauge solved(?) the stability problem, but at the cost of always needing custom rolling stock, but more importantly, making Bart build-out significantly more expensive and unable to take advantage of existing track. That hurts the viability of the Bart ecosystem.
Why not use Indian rolling stock? Modern Indian metro trains are quieter and more comfortable than BART.
(It is the widest gauge in use of heavy-duty mainline railways in the world, but that's a separate thing).
For comparison to other US rapid transit systems (almost all others use 1,435mm/4'8.5"), see table in https://en.wikipedia.org/wiki/Rapid_transit_track_gauge and scroll down to "United States". Or else https://en.wikipedia.org/wiki/List_of_United_States_rapid_tr... which does not list gauges.
[0]: https://en.wikipedia.org/wiki/Track_gauge_in_the_United_Stat...