Yah it's BART that uses an odd track gauge. Although I did read a report saying they did it to make the trains both lighter and also able to withstand wind shear in certain parts of the system.
To be honest though, I didn't find that report very compelling and they didn't back it up with actual load calculations. You really don't hear standard trains being blown over and the Bay Area isn't exactly famous for "high winds" anyway
Mainline BART. e-BART uses standard gauge diesel trains and the half billion dollar Oakland Airport shuttle is a completely bespoke cable car monstrosity.
withstand wind shear
Specifically on the Golden Gate Bridge. Which may or may not be true. The track gauge is perhaps the most standard thing about BART cars.
To be honest though, I didn't find that report very compelling and they didn't back it up with actual load calculations. You really don't hear standard trains being blown over and the Bay Area isn't exactly famous for "high winds" anyway