The problem with BART is that it runs 4 out of 5 lines on the same single stretch of track through SF. So even with the most modern signaling, there will still

The problem with BART is that it runs 4 out of 5 lines on the same single stretch of track through SF. So even with the most modern signaling, there will still be 1/4th the maximum train frequency after the individual lines split. The Pittsburg-Bay Point line counteracts this during rush hour with trains that come just into the city and then turn around, but it's still only an average of ~7 mins between trains. Compare with the Victoria line in London which I believe runs 36 trains an hour!