I learned recently, there used to be trains on the Bay Bridge! https://www.sfgate.com/news/article/50-years-since-trains-cr... I'm surprised there's not an easy rule of thumb: if a road has over two lanes in each direction, the route is busy enough to sustain trains/light rail.
Once upon a time, this line was part of an electric railway connecting Oakland and San Francisco to Sacramento. A hundred years later, electrification between Sacramento and the Bay is a long-term wish list item for Capitol Corridor. It is such a shame that the US allowed its passenger rail system to atrophy.