I took a car train from Hamburg to Vienna last year. Drive the car into the train, and get a sleeping coach for the 14 hour journey - as comfortable a journey I could ask for. Except it cost about €1000 if I remember correctly for my family. I still preferred it because it negated the need to rent a car in Austria, and also saved in baggage costs on a flight. However, if trains could be scaled up, the costs would come down and become competitive even for people who aren’t needing those perks like baggage and car transport.
The train (and tracks) are not as compatible across Europe as I thought. You can travel from Denmark to Austria by train, but be ready to change from Danish train to German, to Austrian train because of different track or signaling systems. Now, the anxiety is on the passenger to not miss a connection at each of those countries (sometimes at the middle of the night).
If EU regulated the train system as a continent, and brought in a whole “European rail system” and not just a ticketing system that works with individual country systems, that would be the beginning of rail as a great transport option. I still have hopes that it’d happen. I just wish it will happen sooner and in my life time.