This train doesnt need a reservation you just board and find a empty seat. Worstcase you have to stand.
Its just ilke the GO-Ontario etc trains-not like VIA. Applies to about anywhere in the whole of EUR. There is NO controlled access and in these trains its completely IMpossible to reserve.
BTW-there are also BUSes- ev 15./20 mins along this route-cost is 1€/preboard or 1,50 by driver and they offer many more views.
If use the better planner of bahn.de/.com it will clearly show if a train needs mandatory REServ or not.
This route is part of a frequent local service along the Cote d’Azur and served by double decker trains - you’ll only ever have to stand on the busiest peak hour trains. Monaco station can be a little confusing as it is buried in the cliff and is multi level with entrances/exits on different levels. More info is here, with a station map .pdf available at the bottom of the page. https://www.garesetconnexions.sncf/fr/gare/frxmm/monaco-monte-carlo/plan-de-la-gare