Bay Area to Los Angeles: train-only route?

Years ago, I rode the train to Los Angeles from the Bay Area but they made us switch to a bus part of the way. I thought there was a train-only route (Coast Starlight), but when I try to book online (trying various Bay Area stations for departure), it says it's mixed service and shows a picture of a bus, then when you click "trip details" it also indicates a connecting bus.

I have not had good experiences on Amtrak buses before and would rather stay off the highways. Is there a way to stay on the train the whole way? I don't particularly care which Bay Area station I leave from.

Thanks!