I found European train tickets between popular/useful destinations to be just as expensive as airline tickets. And, generally, airline tickets were cheaper if

I found European train tickets between popular/useful destinations to be just as expensive as airline tickets. And, generally, airline tickets were cheaper if you could put some advance purchase days into your schedule. When we were touring Europe (pre-Covid), we never had a city-to-city transition where the trains were significantly cheaper than air travel. I suspect this is a function of the popularity of the cities being connected.