You get your reservations by email most of the times and you can show the PDF on your phone or via APP of the train operator. Sometimes you need to print it. More informations at booking. Sometimes you have to print the reservation at a ticket mascine or get it by mail on paper 

Or you just buy a reservation from the ticket office and you have a printed version automatically.

Very often-on trains that do not have MANdatory REServ noone cares about to see it. On overnight trains in special sleep/couchette the car-attendant often wants to have a piece of paper so s/he can do the checks for you to sleep peacefully. AND wake you up in time if you do not go till the far end!

In FRance with this national society that strikes and runs trains-most often now on the platform before boarding there are gates for TGV and/or checks.