I'm home! Trains are great, but four of them to travel ~1200km in ~11 hours, via four cities, in a journey to get from a small village in one country to a small village in another is a little tough if you can't afford any stops along the way.
A lot of the friction points would be resolved by scheduling, booking or infrastructural improvements to rail.
It's obviously hugely popular, despite costing so much more than air. I was able to make it more affordable and easier to split the cost by using Interrail passes
Unusually, the UK's handling of Interrail is amazing - no extra cost seat bookings, just spawn a ticket and off you go. (It is, however, almost impossible to make such a booking if you need one, as the only caveat - a post-sale booking code can be obtained but not fixed seat numbers.)
GWR in the UK has poor respect for standard class passengers on its intercity trains. Nice livery, though.
Back at home, SNCF clearly hates Interrail (you have to even book via SNCB!) and - as a corporate body - doesn't much like its passengers, but staff are consistently amazing. (Note that most regional services in France are less problematic.)
Eurostar clearly does not understand Interrail/Eurail, charges too much for seat bookings (again obtainable via SNCB) and then runs on the honour system, allowing access solely on the €30 seat booking confirmation and never requiring the actual ticket, although I got to see that they do have electronic records of Interrail bookings when I made a specific enquiry at the point of entry.
Almost everyone needs to dramatically increase available seats and space (one beloved rural line I used in Devon had it right and almost no one else).
Points of friction in train travel outside the EU should be reduced, most notably enclosed waiting areas with little space and few food options.
(It is still not great being vegan at Gare du Nord's Eurostar waiting area for the UK. The other side is marginally better, but not by much. Security theatre should not prevent me from buying lunch.)
Trains are by far the best way to travel but it's shocking how poorly intercity and international train travel is treated