Golden Gate Bridge might be possible, but that's quite a climb to bridge level.

Originally the bridge was designed so that rails could be run below the main deck. After the Tacoma Narrows collapse the bridge was stiffened but a space was still left, however the increase in weight made such an idea impractical. Later the bridge deck was replaced and that lightened the bridge. While the work was being done you could actually see the "S" bend in the bridge where the new deck part rose higher.

It is still possible? I don't know. Most modern rail proposals have been more direct routes in tunnel. The main problem is that there is now more commuting intra-county and between Sonoma and Marin counties. SMART is designed to improve transportation from San Rafael north. Connections to The City are secondary.