Silicon Valley might be too low density to support transit inside the city, but is it too low density to support good commuter rail back and forth with SF? I don't think Silicon Valley is less dense than say Westchester County, NY, yet the latter manages to support a pretty great commuter rail network with three lines and 300,000 daily riders within a commuting region of maybe two million people.
Commuter rail cannot exist in a vacuum without local transit. Metro North gets you into Grand Central, but most people don't work right on top of Grand Central - a supplementary transit system is necessary to get people to their actual endpoints.
The CalTrain, which is the closest SF Bay equivalent to MTA commuter trains, stops in San Francisco, but far from local mass transit connections (either MUNI light rail or BART), to the point where there is an industry for private shuttles. This is a significant barrier to the adoption of commuter rail for people who work in the city but want to live further out.
Similarly, once in the Peninsula or South Bay, the CalTrain is often not near a mass transportation connection. Some of the major tech firms are located "near" CalTrain stops, but nearness is often at automobile-scale, as opposed to walking or cycling scale.
You need a pretty-damn-good local transit solution to support commuter networks.
The problem is that the Valley is a host of municipalities of differing sizes, with San Jose at the bottom end. It doesn't really go as far north as San Francisco (yet). Westchester works as it does because it's largely feeding into New York's superb infrastructure -- there's no comparable network (or city of comparable size) on the Peninsula. There are a lot of people commuting from e.g. Belmont to Cupertino or Mountain View to Palo Alto.
