With EURail Global pass - If I get on a train without reservation in Italy - Do I have to pay fine onboard or do I just need to pay the reservation seat surcharge? Does it differ from 1st class to 2nd class?
How does this work in Switzerland, Italy to Swiss or Swiss to Paris?
Here is a quote from the Railpasses section on this site: "You can also get reservations in Europe at train stations or at travel agencies there, when passholder space is still available. Pay before boarding for any required fees, or the conductor will charge more en route."
The quote makes no distinction between different countries.
Normally, trains in Europe cannot oversell, as train tickets are unlimited in number and not specific to a certain departure. For certain premier trains, the railways want to ensure a certain level of comfort and want to limit the number of passengers to the number of available seats, so that passenger are not standing in the aisles. They do this by requiring reservations on certain trains. Therefore, "reservation required" trains do not accept standing passengers. If you have tried at the station to get a reservation, but got the response that the train is sold out, you are not allowed to board. If you do so anyway, expect to be kicked out at the first station where the train stops. If you arrive at the platform without reservation (e.g. because you arrived outside ticket window opening hours) you might be able to get a reservation from the conductor for a surcharge if the train is not oversold. But ask the conductor at the platform and get permission to board before boarding.