Last fall Mrs. ND76 and I were on the NS International train between Brussels South and Schiphol Airport; we had first class tickets (we took this train because it was 50% cheaper than the Thalys high speed train). There were only about 10 people in the car until we got to Antwerp. Apparently the rest of the train was packed, and the people who couldn't get seats sat down in our car, which was almost filled up. The conductor came in and threw out all the poachers, and they promptly left and assumed standing positions in the other cars.

In England, the the train staff catches you in a first class compartment and you don't have a first class ticket, if you are lucky you just get asked to leave; the train staff can demand the full first class fare, and if you don't have it, the police will come and arrest you at the next stop. I've personally witnessed this.