The Center for Clean Air Policy in LA, and Center for Neighborhood Technology in Chicago. has produced a report (January 2006) to better understand the some of the potential impact and value of high speed rail in the U.S., in particular the impact on Greenhouse Gas emissions. There models show there could be a signigicant savings depending of course on how many people switched to this mode of travel. They look at proposed high-speed rail corridors throughout the country. They also identify another area of research to better understand the potential benefits of a network as opposed to separate corridors.
One of those corridors is, of course, between San Francisco and Los Angeles.
You can read the report here: