It is 3rd & Townsend. The depot was built c1914 as a temporary structure for the PPIE (Panama-Pacific Exposition), the plan being to have the "real" depot by continuing up 7th to Market St. with a "Union Station" across Market from City Hall, which would include the WP and AT&SF (I've seen maps of the proposed WP line from San Jose to SF). The other site was inside the horsehsoe of the general office building at 65 Market St., like 6th & Main in LA , which was practise for this SF proposal. However, the City and County said "elevated or subway, no more grade crossings". So 3rd & Townsend lasted, temporarily, for 60 years. The new depot at 4th St is now 30 years old. As they say on the rr, time flies when you're having fun.