Well, and it seems probable that some investments could get a better runtime than Amtrak presently has Los Angeles-San Bernadino, too. I think 4.5 hours Union Station to Las Vegas might be a bit too long.
Practically speaking, I have to wonder what could be done on the Palmdale side of things (which implies a different catchment area).
I disagree that it would be too long. Currently to get between Las Vegas and Los Angeles, you either have to fly, take a bus, or drive. Taking the train is much more convenient than flying, much more comfortable than a bus, and of course unlike a car, you don't have to drive. So I think the convenience and comfort of the train could easily compensate for any extra running time.
To compete on time, a train has to be significantly faster than friving. This is because driving time is measured door to door whereas train times don't include the time it gets to and from the station at either end, plus any waiting time at the station. You typically don't turn up at a sttaion with second to spare, so to make the comparison totally fair, that has to be accounted for too.
This goes for airlines too.
Now, personally I believe that trains have other advantages besides speed, and that there are plenty of people who wopuld take the train even if it took longer, as long as that excess time stayed within reason. For example because the time you spend on a train can be used productively whereas driving time is essentialyl wasted time. But if you are going to compete on speed alone, then you need to compare like to likes, and that means the train must be faster than driving to be equal, and must be a lot faste rto be faster.