in Brest (Ukraine) the stock is lifted and bogies replaced with passengers on bord.
On modern Talgo trains, the gauge changes automatically on the move, but back then it would haven been bogie swap at least on the Polish-USSR frontier.
The earliest I remember was around 1986, an FS over night service from Roma Termini to Milano Centrale. A seat compartment with pullout seats. Three of us slept, the fourth used the emergency seat outside, taking turns.
Since I am not from the UK and its partially European, my first UK journey...