But yeah, for intra-Schengen you just go to the platform a few minutes before departure, find where your carriage will be, and step in once the train comes to a stop.
So much less faff than air travel, and I’ve noticed better pubs at major stations meaning waiting for your train isn’t as boring as it used to be…
Cars and vans in the tunnel don't go through metal detecters, and you can smuggle a far larger bomb that you can in a suitcase.
But eurostar is the exception to the rule with security. I tend to aim to arrive at my mainline station about 5 minutes before the train leaves incase I get delayed on-route, but that's because I live in a small town which only has 1 train per hour. When I take a route with 3tph I don't bother looking at the times, just go when I'm ready and accept on average I have to wait for 10 minutes.
A train station in downtown LA or SF would be inconveniently located for most people in these cities.