Yes.
The train speed is mostly a function of track quality. (also curves and vertical profile)
Trains, even a current ones, are able to travel up to 150mi/h. (Acela top speed) which would be acceptable in the SF to LA distance of about 400 mi. (about 3 hours).
Since the flights takes about 1.5 hour and one has to be there like 1-1,5 hours ahead, and then wait for checking and check out luggage’s, the small seats, the noise of planes.
3 hours of train would be definitely competitive with airlines and beat the cars.
So radical upgrade of existing tracks would be probably quite competitive.
Unfortunately, I doubt that Union Pacific is ready for radical upgrade of their existing tracks.
Without that, Dreamstar could be maybe able to speed up to 30-40mi/h. Clearly uncompetirive with either plane or Car.
So we need this monstrosity of high sped train, which will never happened.
My point is that the FRA overregulates, particularly on the passenger rail side of things. Not only when it comes to track geometry, but also to what they call “crashworthiness” of railcars. That’s why it becomes too expensive for a private company to operate (unless they are politically well connected, like with Brightline); this is by design, so that only government agencies run them, and only as much as they feel like operating.
Private railroads were attempting to develop high-speed trains on their own, before they were not only hit with punitive regulations and taxation, but federal and state governments gave regulatory and tax breaks (and even tax subsidy) to competing modes at the same time.