> existing infrastructure (roads) isn't going anywhere (no need to build anything)

I think that this is a sunk-cost fallacy. Sure, you would need to build infrastructure for trains, but after that trains are now reasonably automated, can carry cargo, and do other things with the exception of stopping at the exact destination you wanted to (in other words, it requires predetermined stops which people outside the US shrugs and just walk or bike the last feet). Also, they're proven to work: even China (which previously didn't have trains to its far-flung places) and they've done what you've expect. I think that the sole reason that anyone wants to invest in automated (non-train) driving is because trains are boring while AI is oh-so-shiny.