BART is looking into creating a second Transbay Tube, but because it's so early in the process, it's not clear if an actual tube will be the end result.
The transportation agency on Wednesday announced that planners are calling the idea a so-called "Transbay Rail Crossing."
At a meeting Thursday morning, BART directors will "see that a companion to the existing Transbay Tube could include both standard width rail tracks (known as standard-gauge) and the wider BART tracks," according to the agency.
Utilizing standard-gauge tracks could help BART connect the Bay Area with Sacramento and other central valley communities, according to the transportation agency.
Planners have listed six objectives for the project, including the possibility of 24-hour service and increased capacity.
BART is looking at starting construction for the rail crossing in about 10 years.