I can't speak to BART, but in Vancouver when people ask why our trains don't run 24/7 the answer is always "because we need maintenance windows". Maybe the sec

I can't speak to BART, but in Vancouver when people ask why our trains don't run 24/7 the answer is always "because we need maintenance windows". Maybe the second transbay tube would allow them to keep one tube open while they do work on the other tube?