I have a 2nd class Eurail pass. I am looking for a train from Interlaken to Paris. And in the Eurail site, part of the journey is Reservation optional and another part is Reservation mandatory. I know it seems obvious, but I want to confirm. In the reservation optional part of the journey, do I just go and sit on any seat in the train?
EDITAR
The reservation optional train is IC 1058 INTERCITY from Interlaken to Basel SBB
Yes, for a reservation-optional train you can sit anywhere that
It varies from railway to railway how easy it is to tell whether a seat has been reserved, or for which part of the route of the train. Sometimes that are little paper reservation markers you can look for. Others have electronic reservation displays for each seat, which may be of varying helpfulness (for example, in Danish IC trains the displays appear to default to MIGHT BE RESERVED for all seats that are not definitely reserved, but at least one can avoid the ones that do show a reservation).
In general the worst that can happen is that someone who has a reservation for the seat you've chosen shows up, at which point you'll have to give it up (and might have to stand if all the other seats are taken by then).