Depends on the countries
Germany doest bound their Tickets on a train Because of that you are quite flexible as no reservation needed but it can Happen that then a lot people wanna travel on the same train. Usually then the 2nd class is crowded in 1st class I only had it between Cologne- Frankfurt & Berlin- Prague.
Generell answer its a differnt System of ticketing and they cant oversell (As Tickets are not fixed on a train).
Thanks for you quick reply!
I understand it may be difficult to answer this question for all countries.
But it would be strange to buy a first class ticket and then have to stand in the aisle …
Anyway, have a nice summer!
I,ll keep this question open for others who might want to share their experiences