Thard58 how do I reserve a spot for a train departure that I want?
You do not need to do this. Trains are mass transit. All you need to do to be on the train you want to be on is be on the platform a few minutes before it is scheduled. Then you get on, and just find yourself a seat.
Sometimes it happens that more people board a train then there are free seats. In that case not everyone will be sitting. But that is rare. If this happens to you it will only be for a short time, as trains make frequent stops, and seat turnover is high, so a seat will free up soon.
So do not overthink this. You have your pass, and with that you just travel.