Well a high speed rail like the TGV going ~300-350km/h would cut that down to roughly 2.5 hours, admittedly that is still higher than going by air, but a lot better than 12 hours.
Flying isn't any faster than 2.5 hours once you figure in the extra time at the airport.
In any event I would be happy to take a conventional train if took less than about eight hours. I don't like flying and the drive is grueling, particularly in traffic.
The problem is AFAIK Amtrak doesn't own any track on the west coast. It's all single track, routing is optimized for freight, and passenger trains have a lower priority than freight trains.
