If memory serves, the substation in the vent at the Ferry Building, was not usable for several days. But not because it was significantly damaged by the earthquake. The problem was at BART's Valencia St substation, several miles up the line, where the 34.5kv source for all the substations on in the Westbay, was fed by an adjacent PG&E substation. That substation was heavily damaged. Thus the power for all of BART'S West Bay substations, and for much of the city as well, was out for quite some time.

That was just as well, as BART engineers had to do extensive structural inspections and testing to make sure that the Transbay Tube was safe, before trains could run through it, anyway. But that was completed before PG&E had power up.

An attempt was made to run trains on the 1kv dc fed from the west vent substation, but results were not very satisfactory, as there was too much line loss in the third rail over that great a distance, especially with multiple trains. So until westbay power became available; what service was offered, was extremely limited, and kept very slow. But that wasn't for very long - as PG&E did a "Bang-up job" cobbling up a temporary bypass at Valencia St. to get BART running at full service again.