San Francisco Bay Area Rapid Transit System: San Francisco Geary Line and Former Marin Line (1961)
From October 1961 Supplement to the June 1961 Engineering Report to the San Francisco Bay Area Rapid Transit District—Four County System.
The thing I didn't quite catch before is that the Geary Line would have been routed into the Transbay Tube, while the earlier Marin Line would have had no track connections to the rest of the system.