People easily tolerate 3h on a plane, and trains are much more comfortable than planes (more space, free to roam).
And you don't have to get to airport, get from airport, check in, wait for baggage.
I would say 5 hours should be non-issue.
"As a rail enthusiast I would love to see trains to Berlin or Milan or even Moscow. But realistically that is many years away."
Remember there's already a direct train connection between Moscow and Paris.
Most people choose to fly these days, of course.