It depends a lot on where you start/go to, I've recently experienced more and more controls before boarding, with gates, police, sometime bag checks, &c. I assume it's due to more and more migrants riding trains for free to reach some countries, I see it almost every time I take long range train between France/Germany/Luxembourg
It happens more and more due to illegal immigration. I've beens topped in trains, buses, highway parking, by police officers asking of passport and reasons to travel.
In Paris/Marseille/Strasbourg I've always been controlled since ~2020
From the airport straight into the train?
Yes sometimes there are intra-Schengen checks in trains but the most I've seen is people checking the passports in route
It’s unlikely they have border controls since then although they probably did before.
Schengen allows free movement of EU citizen and people legally residing/visiting.
It means you can travel without visa.
It _does not_ mean borders are fully open and you can go in and out with absolutely no control
(And also in China, though there they also have similar checks at some metro and other railway stations.)
Also bus lines in mexico do similar checks.