score:7
I hope you do not plan to travel soon as there is work on the track each Saturday in March 2017 (according to my source.)
Not only do you not have to make reservations for this train, it is impossible to make reservations. There is no system of reservations on this train, as indicated on that same source.
If you need to travel in March, there are alternative trains, but it takes several changes of trains. Or it is possible to take the fast train but you need a reservation, it is rather expensive and there are (as far as I know) restrictions on numbers of passholders. If you are flexible in which day you travel you might get around the works by going on a weekday. (But do double check for the day you actually want to travel.)