Same situation when travelled in Germany/France/Luxembourg - most of time have to point person to overhead LED that this is seat you have booked (if electronic sign available) or ticket with your seat number on it. Some older trains in Germany have simply 'reserved' on sign - no name tag. Especially this time of year when large amount of Inter railers going around people sit any where and great craic listening to irate germans trying to explain to eighteen year old in broken english get out of my bloody seat
That said most people have no problem getting up as realise people can reserve