Ever been on trains? What you suggest is impossible- a RES means that a seat is allocated to you. This is mandatory on in fact only a very few trains-except that about each&every newbee seems to want to ride exactly those trains and then finds its hard to make such REServ.

Trains with only optional or NO RES are always open for anyone to board-its not a plane with check-in, gates, seatbelts, stweardess,  etc. Similar to any metro/subway in USA or the NwJersey commuter or Toronto GO or CalTrain. It also seems any newbee has immense fears about full trains-most of this boils down to the limited quota on some trains-mostly FRench hi-speed.

The Eurail pass is your ticket.

For some trains you additional must have a seat reservation. On those trains you are not allowed to stay without a seat reservation. These trains are night trains and many high speed trains (not within Germany) and many trains in Spain.

Then there are many trains with optional reservation. Here you can travel with only your pass as a ticket. However, during high season it might be wise to make a seat reservation to be sure to get a seat. There are trains, at least Inlandsbanan in Sweden, where you only can travel without reservation if not all seats are reserved by others.

At last there are trains where you can't make a seat reservation.