There isn't really any data backing up this claim.
https://ec.europa.eu/eurostat/web/products-eurostat-news/w/e...
Planes are much popular and this data seems to include commuters so the proportion of people travelling medium to long distances by train is much lower (and most medium/intercity travel is obviously usually done by car and not trains..)
Amongst other significant issues (e.g. cross border travel) high-speed trains tend to be quite expensive, so unless you're travelling alone a car is usually cheaper.
https://www.ryanair.com/gb/en/lp/promotion/getaways
That page shows me flights to Bulgaria, France and Italy for around €25.
It is more like €50 if you aren't flexible, and more again if you need business-friendly times.
Ryanair fly to the primary airport in eastern and southern Europe. I'll take a direct Copenhagen to Sofia flight rather than enjoy changing with Lufthansa.
I feel like the premise of a train is that it costs less than a plane and uses less fuel per passenger, so it should be slower but have the countervailing advantage of being less expensive. That is not what I'm observing.