Belgium has lots of loco hauled stuff running around on their network.
The local and regional trains (L- and S- trains) are operated with units, many of them being Desiro MLs, on the longer distance routes you're very likely to find HLE 18s, HLE 21s and HLE 27s pushing or pulling coaches. The Belgian railways' M6 double decker coaching stock is very comfortable.
On HSL 2, the high speed railway between Leuven and Liège, loco hauled trains travel at speeds of up to 200 km/h. Look for trains traveling between Oostende and Eupen, those go via HSL2.
You can view the booked formations of Belgian trains on this website:
BeLuxTrains in english