I had to be at the international terminal at St Pancras in London a half-hour before the train left, though I think some of the more expensive business-class tickets only require you to be there 10 minutes early.
Getting in meant showing ticket, passport, and some routine security scans, the latter being quick and not nearly as intrusive or theatrical as typical modern airport security. (I’m a rather private person, and just the possibility of being put through one of the virtual strip-search machines at an airport is enough to make me not want to fly anywhere any more, even without all the other hassle and time required.)
On the train, there probably are some luggage limitations if you read the full terms, but no-one was taking a ruler to my backpack or weighing my suitcase and charging me extra on the return journey just because I had added a 1kg souvenir. My suitcase was within reach/eyesight throughout my journey, and there was ample space for smaller luggage right by my seat throughout as well.
Perhaps the most striking thing of all was that once I was clear of the UK, changing trains at Brussels (from the international Eurostar from London to an also-international Thalys to Amsterdam) was as simple as changing local services back home. The second train literally pulled into the station a few minutes before it was due to leave, I just went onto the platform, and the only fuss at all was because the train had reserved seating so someone helpfully checked my ticket and pointed out where my seat was as I boarded.
I haven’t travelled between many different places within continental Europe (as distinct from travelling between the UK and a European destination) but if that’s the norm within the Schengen Area then we’re doing it wrong. Transport systems should be primarily about getting the travellers to where they want to go efficiently and pleasantly, and most of Europe appears to be much better at those things than we are.