International HSTs with passport required border crossings (ie Schengen-UK) have passport control and security - I think the security checks are to minimise the risk of terror attacks in the channel tunnel though.

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…