The busiest line for freight is between Aarschot and Hasselt (line 35), although gaps of 2 hours between freights are possible. Diest has a direct hourly service to Brussels and coming from Brussels you have a perfect connection with the local service in Aarschot. If you don't mind walking a few miles, leave the train at Diest and return to the small station of Zichem on foot. There is an excellent trail next to the railway without barriers, so spotting and photography are easy. The local stations at Langdorp and Testelt also offer easy access to well known photo spots.
Other lines with an important freight flow are Antwerpen - Gent, Gent - Mechelen - Leuven and Antwerpen - Roosendaal (NL). On that line, I recommend the station of Antwerpen-Luchtbal where you have freight trains to and from the port, trains from the Netherlands but also high speed trains (Eurostar and NS Eurocity Direct) as well as the SNCB Eurocity service to Rotterdam (with class 28) and European Sleeper from Prague on monday, wednesday and friday.