I know it sounds absurd, but you can check Google Maps for the possible routes and times.[1] It looks like a Greyhound bus is actually 2 hours faster (16 hours). Also it looks like some parts of the track along the west coast are undergoing maintenance. If you want to take the train down to San Diego you have to get off your train, take a bus, then get on another train.[2]

1. https://www.google.com/maps/dir/Portland,+Oregon/Oakland,+Ca...

2. https://www.amtrak.com/alert/pacific-surfliner-coast-starlig...