A few years ago on a train between Austria and Italy, a group of border guards got on the train.
The train was made up of compartments. When the guard came to my compartment, they first interviewd the guy sitting across from me. He had a German passport but looked more middle eastern. He was questioned a bit. Then the guard went to the couple sitting next to the window. They also looked middle eastern and got questioned even more.
Then it was my turn. I started to hand him my US passport, he took a look at is as I held it, waived his hand as if saying "not necessary" and left the compartment.
A couple of years ago in southeastern France, a group of police officers got on the train and walked through. They didn't seem to talk to anyone. When we got to Carcassonne, they went through the station asking specific people for their identification. I just left to get a taxi.
The worst was in Stockholm two years ago. Arriving on a flight from Athens, everyone had to go through passport control. I was grilled for about 10 minutes about my plans. I had to show my air ticket out of Stockholm to Helsinki for a few days later and my air ticket out of Schengen. Plus a lot of questions of why I was there, what were my plans, etc. In all my years traveling anywhere, this was the most questioning I had ever had. And I was arriving from a Schengen country. I wasn't prepared for any of this as I didn't expect passport control.