The 10:31 trip leaves only 5 min to change from Luxembourg, Gare Centrale (Tram) to Luxembourg, Gare Centrale (1 min walk); a delay could make you miss line RB.
1st class
1st class on trains is not free; buy a 1st-class ticket (CFL app or ticket machines) before boarding.
Free travel
Public transport inside Luxembourg is free since 1 March 2020: buses, the tram and trains in 2nd class need no ticket.