the validity of a Brussels to Luxembourg ticket's route is determined by the borderpoint, on normal Brussels to Luxembourg tickets this will be Sterpenich Frontiére, thus via the direct line Brussels - Namur - Arlon - Luxembourg (lines 161 en 162), If you want to travel via Liége and Gouvy (lines 36(N), 2, 43 and 42) you have to buy a ticket via "Gouvy Frontiére" (because this is a longer trip and thus more expensive).
Equipment used on the line is normally a CFL electric loco of the 3000 series (same as the NMBS-SNCB 13 series) and 3 (sometimes 4) NMBS-SNCB coaches, normally 1 I10 first class (coach seating, no airconditioning, Vmax 160kph) and 2 to 3 I10 second class (coach seating, no airconditioning, Vmax 160kph). The I10 coaches on this relation are sometimes replaced by the older I6 coaches, featuring compartment seating and airconditioning, sometimes even a first class coach used as second class coach.
The route itself is completely electrified, 3KV DC Liers-Rivage and 25KV AC Rivage-Luxembourg.
Between Liége and Gouvy you have some extra peak-services run from Brussels with a NMBS-SNCB series 13 and 12 I11 coaches and some local school service trains run with bitension AM96 units. After Gouvy you will find a lot of CFL trains, mostly run with double-deck stock.