If you're convenient to the airport on both sides and want to spend on both the flight and the room, knock yourself out. No 800 sq ft suites with balconies on a train. But if your aim is the city center on either end, and you'd like to not waste time or money, a night train with comfortable bunks is perfect. You waste few waking hours traveling in/out of airports, stripping for TSA, etc.

The odds of either origin or destination being near the LA train stop are roughly 0% and maybe 1% on the SF side.

This is just as inconvenient for the vast majority of residents who make the LA/SF travel as going to the airport.