"Only a second Transbay Tube would allow BART to as much as double the number of trains it pushes across the bay. This would require constructing a new set of tracks to separate some BART routes from the existing tracks before the Oakland Wye (the junction point between the downtown Oakland and Lake Merritt stations and the West Oakland station, where all routes converge into one main line). It would also require a second BART alignment through San Francisco so that trains using the new Transbay Tube could remain separated from the existing tracks."

From SFGATE.COM