At the time it was being designed, press reports said that the BART broad gauge track was the result of extensive wind tunnel testing of scale model trains. The desire was maximun stability on elevated track with prevailing Bay Area winds. I believe most other railfan musing falls in the category of urban legends.

The original BART Concord test track was used to evaluate different technology for powering and computer control of trains. Several different AC and DC voltages were tried. 1000 volt DC third rail power was determind to be the best compromise. With the cheap reliable solid state rectifiers we have today I suspect AC power would have been supplied to the trains. Although three-phase AC power had been used on several transit systems in Germany (the power collectors were monstrosities), the problems of getting three-phase power into a moving train might have prevented using it for BART. Three-phase power is much better for converting to DC than is single-phase AC power. AC third rail power also causes problems with regenerative braking.

BART did not want to be limited to existing technology. 1000 volt DC motors were available in the 1940's. Transit operation could not increase voltage because of the large amount of existing 600 volt equipment. Ohm's Law applies. The higher the voltage, the lower the current for the same amount of traction power. This allows for less coper wire used for traction power feeders. Third rail also requires much less feeder wire than various overhead wire systems.

BART was meant to improve the state of the art. However the cars were not as reliable as had been predicted. So the system was built without side tracks to park dead trains temporarily. That problem plagues BART to this day.