Ever been on a cercanias in ESpana/RENFE? Thats how it goes. You must always add trip to tripreport.

Ever even been on a train except AVE? You may have noted that there is no access control-people get off, people get on. If its superbusy, as in the peaks and you are too shy to ask some egoistic one to take bags from seat-then you stand. At every stop people get off and on again.

If you are confused as to which trains require seat reservations and which do not, you can use this site, but as an alternative, the UK-domiciled www.seat61.com can give you much useful information.

If a train has optional reservation, like all domestic day trains in Germany, you can travel without a reservation and just find a free seat. If there are no free seats you stand or sit on the floor. 

However if the train is too crowded, the staff might, for security reason, ask people without reservation to leave the train. This is unusual but I have heard that it has happened in Germany and Austria. 

There are trains, like Inlandsbanan in Sweden, where reservation is optional, but if you don't have a reservation and the train is fully booked, then you are not allowed to board the train or you might have to leave the train. This means that as long are there are free seats you can travel without a reservation but if all seats are reservation you need a reservation. 

Where do you plan to travel?