It was always my understanding, that the broad gauge was chosen because BART wanted the public image of not being a railway at all. And if anyone still made the connection (duh), it had to be very unlike any other railroad.

In fact, in the very early days, if you called it a railroad or railway within their earshot, you'd get your head handed to you in a porcelain box. Political research had told them that to get the property tax bonds passed, they had to overcome the extremely negative image of rail passenger service, as created by the Southern Pacific. It could not in any way look like the SP. And since most people knew very little of any other railroads, it couldn't be like them either - they were all alike. Thus it was a "Rapid Transit" - NOT A RAILWAY! And was to be different in every way possible.

As to the purported added stability of broad gauge, it only increases the "turning moment" needed to cause a roll-over. But that is only applicable at about twice authorized speeds around curves - hence irrelevant.

Broad gauge was a purely political decision!